In a medium-sized mixing bowl or bowl of a stand mixer, mix together sugar, cake flour, baking powder, baking soda, and salt.
Add butter and mix on medium-low speed for three minutes. Because there is so little butter, you'll end up with a very fine crumb texture.
In a small mixing bowl, whisk together eggs, sour cream, oil, and vanilla extract until smooth.
Add the egg mixture to the flour mixture and beat on medium speed until just combined.
Slowly add milk and mix on low speed until just combined. The batter will be liquid. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)
Divide batter evenly between 12 baking cups. They should be about 1/3 full.
Lemon Filling Instructions
Preheat oven to 350 F.
In a medium-sized saucepan, whisk together sugar, cornstarch, flour, and salt.
Slowly add water, stirring until mixed well.
Bring to a boil over medium heat, stirring constantly, and cook for 3 minutes.
Remove from heat and slowly pour half of the hot mixture into the egg yolks, stirring constantly. If you pour too much too quickly you can accidentally cook the eggs - bad news.
Stirring constantly, pour the eggs back into the pot, return to a boil, and cook for another 2 minutes.
Add the zest, lemon juice, and butter.
Cook for another 2 minutes, stirring to ensure that nothing sticks to the bottom of the pan.
Divide filling evenly between the baking cups on top of the cupcake batter. After adding the lemon filling, the cups should be just over halfway filled.
Bake for 18 minutes or until the cupcakes bounce back when lightly touched. They may have divots from the weight of the lemon filling and that's OK. You will be covering the surfaces up.
Toasted Meringue Topping Instructions
Set a metal mixing bowl (I suggest using the one from your stand-mixer if you own one) over a pot of boiling water and fill with egg whites, sugar, and salt.
Stir the mixture constantly for about 3 minutes or until the sugar is dissolved and the eggs reach 110 F. If you don't have a candy thermometer, don't sweat the exact temperature. The whole point is to heat the eggs a little so that they are safe to eat.
Place the bowl on a stand mixer with the whisk attachment or use a hand-held electric mixer and mix on high speed for 8 minutes.
Add the vanilla.
Mix for another 5 minutes or until the mixture forms shiny, stiff peaks.
Use a spoon to pile high onto cooled cupcakes. Have fun pulling up different sections of the meringue to sculpt peaks.
Use a culinary torch to toast the frosting - be careful not to burn the baking cups or yourself.
Store the cupcakes in the refrigerator up to three days and bring to room temperature before eating.