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1
Preheat oven to 350*F.
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2
Grease and flour 12 standard-size muffin cups (or, line with paper liners); set aside.
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3
FILLING: Combine the peanut butter and sugar in a small bowl until well blended; set aside.
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4
CUPCAKES: Combine the cocoa powder and hot water in a cup until smooth; set aside.
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5
In a small bowl, whisk together the flour, baking powder, baking soda, and salt; set aside.
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6
In a medium bowl, using an electric mixer on high speed, cream the butter and peanut butter until light and creamy; slowly blend in the brown sugar.
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7
Lower the mixer speed to medium; blend in the egg yolks, one at a time.
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8
Blend in the cocoa mixture, then the vanilla; blend well.
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9
Lower mixer speed to low.
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10
Blend in the flour mixture alternately with the milk, beginning and ending with the flour mixture and beating just until combined.
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11
Spoon about 2/3 of the batter into the prepared tins.
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12
Top with equal amounts of filling, then the remaining batter; smooth tops.
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13
Bake until tops are cracked, edges are springy, and centers are still soft when pressed gently with a fingertip, about 20 minutes.
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14
Cool in the tins slightly before removing.
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15
Frost or glaze with your favorite topping.
