1) Preheat the oven to 375°F. Grease a standard muffin pan, or line with greased cupcake papers.
2) To make the cupcakes: Combine the dry ingredients, then mix in the butter until crumbly.
3) Whisk together the eggs, milk, and vanilla. Stir the wet ingredients into the dry in two additions.
4) Scoop about 1/3 cup batter into each muffin cup.
5) Bake the cupcakes for 16 to 18 minutes, until they're lightly browned and a toothpick inserted into the center of one of the middle cupcakes comes out clean.
6) Remove the cupcakes from the oven; after 5 minutes turn them out of the pan onto a rack to cool.
7) To make the frosting: Beat together the butter, shortening, and salt.
8) Add the sugar, vanilla, and ¼ cup milk, and beat until fluffy. Add additional milk if the frosting seems too stiff to spread.
9) Use a cupcake corer or small spoon to scoop some of the center out of each cupcake. Fill and ice with the frosting.
Yield: 12 cupcakes.