For the cupcakes:

1. Preheat oven to 325°. Line cupcake pans with paper liners; set aside.
2. In a bowl of an electric mixer fitted with a paddle attachment, combine flours, sugar, baking powder, and salt; mix on low speed until combined. Add butter, mixing until just coated with flour.
3. In a large glass measuring cup, whisk together eggs, milk, and vanilla. With mixer on medium speed, add wet ingredients in 3 parts, scraping down sides of bowl before each addition; beat until ingredients are incorporated but do not overbeat.
4. Divide batter evenly among liners, filling about 2/3 full. Bake, rotating pan halfway through, until a cake tester inserted in the center comes out clean (17 to 20 minutes).
5. Transfer to a wire rack to cool completely. Repeat process with remaining batter. Once cupcakes have cooled, use a small offset spatula to frost tops of each cupcake. Decorate with sprinkles. Serve at room temperature.

For the Buttercream Frosting:

1. In a bowl of an electric mixer fitted with a paddle attachment, beat butter until smooth and creamy, 2 to 3 minutes.
2. With mixer on low speed, add 6 cups of sugar, milk, and vanilla. Mix until light and fluffy. If necessary, gradually add remaining 2 cups sugar to reach desired consistency.