Preheat oven to 350 degrees. Butter two (or three) 9-inch baking pans and line the bottom of each with parchment paper (note: sometimes I like to bake this in three layers; each layer is thinner than if you use two pans, but simply divide evenly between however-many pans you choose to use and all will be well). Butter the paper and dust the pans with flour, tapping out the excess.
Whisk baking powder and salt into flour.
In another bowl, beat butter and sugar until light and fluffy. Beat in the egg yolks one at a time. Add the vanilla to the mixture.
Add the flour mixture and milk alternately, starting and ending with one third of the flour. Stir just until the flour is incorporated.
In another bowl, whisk or whip egg whites to soft peaks. Stir one-third of the egg whites into the batter, then gently fold in the rest. Pour the batter into the prepared pans and bake until a toothpick inserted into the center comes out clean, 30 to 40 minutes.
Make Buttercream
For the vanilla butter cream
(makes about 4 cups)
16 tablespoons (2 sticks) unsalted butter
6 cups confectioners’ sugar
2 tablespoons whole milk, plus more, if needed
2 teaspoons vanilla extract
Have all the ingredients at room temperature.
In a large bowl, combine the butter, confectioners’ sugar, milk and vanilla and beat on low speed until combined, about 1 minute. Stop the mixer and scrape down the sides of the bowl. Increase the speed to medium and beat until fluffy, about 3 minutes.
If the frosting is dry, add more milk, 1 teaspoon at a time, until it is creamy but still holds peaks.
Assemble
Layer the cake with the strawberry jam and lemon curd and frost it with the buttercream.