INGREDIENTS
Cupcakes:
2-1/4 sticks unsalted butter, at room temperature
3 cups sugar
6 extra-large eggs, at room temperature
1 cup sour cream, at room temperature
1-1/2 teaspoons pure vanilla extract
3 cups all-purpose flour
1/3 cup cornstarch
1 teaspoon kosher salt
1 teaspoon baking soda
24 cupcake liners - 2 (12 cup) cupcake pans

Buttercream:
1-1/2 cups milk
6 tablespoons flour
1-1/2 cups sugar
1-1/2 cups unsalted butter - room temperature
1 tablespoon vanilla extract

Directions for the Cupcakes:
Heat the oven to 350 degrees. Line muffin pans with paper liners.
Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment on high speed, until light and fluffy. On medium speed, add the eggs, 2 at a time, then add the sour cream and vanilla. Scrape down the sides and stir until smooth.
Sift together the flour, cornstarch, salt, and baking soda in a bowl. With the mixer on low speed, add the flour mixture to the butter mixture until just combined. Fill the cupcake liners to the top with batter. Bake in the center of the oven for 20 to 30 minutes, until a toothpick comes out clean. Cool to room temperature.

Directions for the Buttercream: In a small heavy pot, heat the milk and slowly whisk in the flour. Continue to cook, stirring out all lumps until a paste like mixture forms. In the bowl of an electric mixer, beat the butter with the sugar until light and fluffy. Gradually add in the milk mixture and continue beating until a light and fluffy buttercream has formed. Beat in the extract and taste for flavoring. Spread the frosting generously on top of each cupcake.