Preheat oven to 350°. Line 3 cupcake pans with liners and set aside.
In measuring cup, combine milk and vanilla.
In a stand mixer, beat butter at medium speed until creamy. Add in sugar until fluffy.
In a separate bowl, sift together flour and baking powder. Carefully add flour mixture to the butter mixture alternating with the milk, beginning and ending with the flour.
In a separate bowl, beat egg whites at medium speed until stiff peaks form. Carefully fold egg whites into cupcake batter.
Fill each cupcake liner about 2/3 full. Bake for 18 minutes or until toothpick inserted in the center comes out clean. Let cool completely before frosting.
Beat butter and shortening on medium speed until creamy. Slowly beat in powdered sugar one cup at a time. Mix in salt and vanilla, brown sugar and milk.
To assemble cupcakes, use a paring knife, carefully hollow out tops of each cupcake. Place a generous spoonful of pie filling into the cavity of each cupcake. Pipe frosting on top.