Aedan's Lemon Surprise Birthday Cake (or Cupcakes)

I've made different versions of this over the years, at Aedan's request. It works great as a layered cake or as cupcakes! 

   

Thick Lemon Curd (can be made days ahead) 
(Makes approx. 1.5 cups) from www.loveandoliveoil.com/

 200g sugar 
 3 Tbsp lemon zest

 6 tsp cornstarch Pinch of sea salt 6 large egg yolks 1.5 large eggs 180g fresh squeezed lemon juice  63g unsalted room temperature butter cut into cubes

» To Make lemon curd:

• In a bowl, rub lemon zest into granulated sugar to release the flavorful oils. Whisk in cornstarch and salt until evenly incorporated.
• Transfer to a medium, heavy-bottomed saucepan set over low heat. Whisk in egg and egg yolks until it forms a thick paste, then whisk in lemon juice until smooth.
• Continue to whisk regularly over low heat until the mixture starts to thicken. You can switch to a spatula at any point if you like. Continue to cook until curd reaches 170ºF and thickens enough that you can swipe your spatula through it and it will not fill in for at least 3-5 seconds.
• Remove from heat and whisk in butter, one piece at a time, whisking until fully melted and incorporated before adding the next.
• Strain through a fine mesh sieve to remove the zest and any bits of cooked egg (this will give you a beautifully smooth texture).

Let cool completely, or, if not using right away, press a layer of plastic wrap onto the surface of the curd and refrigerate overnight or for up to 5 days of freeze for up to a month.  Cake Recipe (from https://preppykitchen.com/lemon-cake/) (Makes 24 cupcakes or 2 8-in round cakes)

 2½ cups all-purpose flour(300g)

 1½ teaspoons baking powder

 ½ teaspoon baking soda

 ¾ teaspoon salt

 1 cup unsalted butter softened (227g)

 1½ cups granulated sugar (300g)

 2 tablespoons lemon zest (about 2 lemons)

 3 large eggs room temperature

 1 cup low-fat buttermilk room temperature (240ml)

 ¼ cup fresh lemon juice (about 2 lemons) (60ml)

 1 tablespoon vanilla extract

 

» To make the Cake OR Cupcakes:

• Preheat the oven to 350°F/170°C. Lightly grease two 8-inch round cake pans with butter or baking spray and line the bottoms with parchment paper. Wrap the outside of the cake pans with soaked fabric baking strips, if you’d like. (This produces flat cake layers!) OR prep cupcake pan with liners
• In a medium mixing bowl, whisk together the flour, baking powder, baking soda, and salt.
• In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy. Add the sugar and lemon zest and beat until light and fluffy, about 5 minutes. Add the eggs one at a time, scrape down the bowl between additions, and beat until well combined and fluffy.
• In a liquid measuring cup, stir the buttermilk, lemon juice, and vanilla together.
• With the mixer on low speed, add a third of the flour mixture followed by half of the buttermilk mixture. Continue, alternating between the flour and buttermilk, mixing just until combined. Stop and scrape down the bowl occasionally. Divide the batter among the prepared cake pans. OR Fill cupcake liners 2/3.
• Bake for 35 to 40 minutes or until the center of the cakes are springy to the touch and the sides are just starting to pull away from the pan.  OR Bake for 12 minutes and check center of one with a toothpick when you think they are done. Let the cakes cool in the pan for 15 minutes then invert or remove and cool completely on a wire rack.

Lemon Swiss Meringue Butter Cream  (from https://sallysbakingaddiction.com/swiss-meringue-buttercream/#tasty-recipes-80765)
 6 large egg whites (approx. 180g)
 2 cups granulated sugar (approx 400g)
 1 and 1/2 cups unsalted butter, softened but still cool to the touch and cut into Tbsp size pieces (approx 60°F)
 2 teaspoons pure vanilla extract  or 1 packet Vanilla Zucker
 1 vial of Dr. Oetkers Citron Aroma (https://www.coop.ch/de/lebensmittel/vorraete/backzutaten/klassische-backzutaten/fruechte-aromen/dr-oetker-zitrone-aroma-4-stueck/p/3030228)
 1/8 teaspoon salt 

» To make the SMBC: Note: this is tricky for the first few times, and how it turns out can also depend on room temp, butter temp, humidity, etc... so review her post and video for preparing and troubleshooting

Separate the eggs: If you haven’t done so yet, separate the eggs first. Separate 1 egg white in a small bowl, then place the egg white in your heatproof mixing bowl. Repeat with the remaining egg whites. This way, if a yolk breaks in one of them, you don’t waste the whole batch.
 

Whisk sugar into the egg whites, then set the bowl over a saucepan filled with just two inches of simmering water over medium heat. Do not let the bottom of the egg whites bowl touch the water. Whisk the whites and sugar constantly until sugar is dissolved and mixture has thinned out, about 4 minutes. The mixture will be thick and tacky at first, then thin out and be frothy white on top. To test that it’s ready, you can use your finger or an instant read thermometer. Lightly and quickly dip your finger (it’s very hot, be careful) and rub the mixture between your thumb and finger. You shouldn’t feel any sugar granules. If using a thermometer, the temperature should read 160°F (71°C).

No need to let it cool down to start this next step—it’s important to begin mixing while it is still warm. Transfer mixture to the bowl of a stand mixer fitted with a whisk attachment (if you aren’t already using the metal bowl that comes with it). You can use a hand mixer instead, but this step takes awhile and your arm tires quickly. On medium-high speed, beat the mixture until stiff glossy peaks form and the meringue is no longer warm to the touch, at least 10-15 minutes. On particularly humid days, this has taken me up to 17-18 minutes. If it’s still not reaching stiff peaks, stop the mixer, place the bowl—uncovered—in the refrigerator for 10 minutes, then return to the mixer and continue beating until stiff peaks form. (This has always worked for me when it’s taking forever to reach stiff peaks.)

If the bowl and meringue still feel warm, wait until both cool to room temperature (around 70°F (21°C)) before adding the butter in the next step. Feel free to place it in the refrigerator. A warm bowl and meringue will melt the butter.

Switch the stand mixer to the paddle attachment. On medium-high speed, add the butter 1 Tablespoon at a time. Wait for the butter to fully mix in before adding the next Tablespoon. After all the butter has been added, turn the mixer down to medium speed and fully beat in the vanilla and lemon Aroma and salt, about 30 seconds.

Your Swiss meringue buttercream should be thick, creamy, and silky smooth and is ready to use on any cake, cupcake, or other confection.