Step 1

Preheat oven to 350 degrees. Brush standard muffin tins with butter; dust with cake flour, tapping out excess. Whisk together cake flour, baking soda, and salt.

Step 2

With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Beat in vanilla. Reduce speed to low. Add flour mixture in three batches, alternating with two additions of buttermilk, and beating until combined after each. Beat in chocolate until combined.

Step 3

Divide batter evenly among prepared cups, filling each three-quarters full. Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, 20 minutes. Transfer tins to wire racks to cool 10 minutes. Run a small offset spatula or knife around the edges to loosen; turn out cupcakes onto racks and let cool completely. Cupcakes can be stored up to 3 days at room temperature in airtight containers.

Step 4

To finish, use a serrated knife (and a gentle sawing motion) to split cupcakes in half horizontally. Spread a heaping tablespoon of frosting on top of each bottom half; replace top halves. Spread more frosting on top of each cupcake. Serve immediately.