For the cupcakes:
2 1/4 cups cake flour
1 Tbsp baking powder
1/4 tsp salt
4 egg whites
1 1/4 cups milk
1 1/2 cups sugar
Lemon zest from one lemon (I used a Meyer lemon)
1 stick butter, at room temperature
1 tsp pure vanilla extract
1 Tbsp lemon juice
For the vanilla bean-lemon buttercream frosting:
1 stick butter, softened
1/2 cup shortening
4 cups powdered sugar
1-2 Tbsp lemon juice
1 vanilla bean, split and scraped