Preheat oven to 350 degrees with the oven rack in the middle position. Line a 12-cup muffin tin with cupcake wrappers.
Combine the flour, baking powder and salt in a medium bowl and set aside.
Cream together the butter and sugar until light colored and fluffy, about 3 minutes.
Add the egg, egg yolks, and vanilla and beat at medium speed until thoroughly incorporated. Add the sour cream and beat until incorporated.
Add the flour mixture and beat until just incorporated.
Spoon the mixture into the muffin cups and bake until a toothpick inserted in the center of a cupcake comes out clean, about 20 minutes.
Remove and cool to room temperature on a rack, about 45 minutes.
Black and White Frosting Instructions
Stir together confectioners sugar, corn syrup, lemon juice, vanilla, and 2 tablespoons water in a small bowl until smooth. If icing is not easily spreadable, add more water, 1/2 teaspoon at a time. Transfer half of icing to another bowl and stir in cocoa, adding more water, 1/2 teaspoon at a time, to thin to same consistency as vanilla icing. Cover surface with a dampened paper towel, then cover bowl with plastic wrap.
With offset spatula, spread white icing over half of each cupcake. Starting with cupcakes you iced first, spread chocolate icing over other half.