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1
Preheat oven to 350F.
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2
Brush standard muffin tins with butter; dust with cake flour, tapping out excess.
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3
Whisk together cake flour, baking soda, and salt.
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4
With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy.
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5
Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed.
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6
Beat in vanilla.
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7
Reduce speed to low.
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8
Add flour mixture in three batches, alternating with two additions of buttermilk, and beating until combined after each.
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9
Beat in chocolate until combined.
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10
Divide batter evenly among prepared cups, filling each three-quarters full.
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11
Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, 20 minutes.
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12
Transfer tins to wire racks to cool 10 minutes.
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13
Run a small offset spatula or knife around the edges to loosen; turn out cupcakes onto racks and let cool completely.
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14
Cupcakes can be stored up to 3 days at room temperature in airtight containers.
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15
To finish, use a serrated knife (and a gentle sawing motion) to split cupcakes in half horizontally.
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16
Spread a heaping tablespoon of frosting on top of each bottom half; replace top halves.
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17
Spread more frosting on top of each cupcake.
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18
Serve immediately.
