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1
Preheat oven to 350F.
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2
Line 24 cups of standard muffin tins with paper liners.
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3
Spray an 8-inch square pan with nonstick cooking spray.
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4
Line bottom of pan with parchment paper; spray parchment.
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5
In a mixing bowl, whisk together flour, baking powder, and salt.
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6
Heat butter and chocolate in a heatproof mixing bowl set over (not in) a pan of simmering water, stirring occasionally, until melted.
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7
Remove from heat, and add sugar.
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8
With an electric mixer on medium-high speed, whisk until mixture is smooth.
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9
Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed.
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10
Add vanilla, and beat 3 minutes more.
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11
Reduce speed to low.
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12
Gradually add two thirds of the flour mixture; beat until just combined.
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13
Remove bowl from mixer.
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14
Add remaining flour mixture, and fold in by hand, just to combine.
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15
Divide batter evenly among lined cups, filling each two-thirds full.
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16
Spread remaining batter in prepared baking pan, smoothing top with an offset spatula.
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17
Bake, rotating tins and pan halfway through, until just set (but still feels soft), and top is shiny, about 17 minutes for cupcakes and 25 minutes for square pan.
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18
Transfer tins and pan to wire racks and let cool completely before removing cupcakes and square.
