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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Line 2 (12-well) muffin pans with paper liners; set aside.
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3
Whisk together the flour, baking soda, and salt in a medium bowl; set aside.
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4
Whisk together the cocoa and boiling water in a small bowl until combined.
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5
Slowly whisk in the milk; set aside.
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6
Place the butter in the bowl of a stand mixer fitted with a paddle attachment and beat on medium-high speed until fluffy and light in color, about 3 minutes.
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7
Add the sugar and vanilla and continue to beat for another 4 minutes.
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8
Stop the mixer and scrape down the sides of the bowl and the paddle with a rubber spatula.
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9
Return the mixer to medium-high speed.
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10
Add the eggs 1 at a time, letting each incorporate fully before adding the next.
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11
Stop the mixer and scrape down the sides of the bowl and the paddle.
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12
Add a third of the flour mixture and turn the mixer to low speed, mixing until the flour is just incorporated.
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13
Add half of the cocoa mixture and mix until just incorporated.
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14
Continue adding the remaining flour mixture and cocoa mixture, alternating between each and ending with the flour, until all of the ingredients are incorporated and smooth.
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15
Stop the mixer, remove the bowl, and stir in any remaining flour streaks by hand, making sure to scrape the bottom of the bowl.
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16
Divide the batter among the muffin wells (the wells will be about two-thirds full).
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17
Place the pans side by side in the oven and bake for 15 minutes.
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18
Rotate the pans front to back and side to side and bake until a toothpick inserted into the center of the cupcakes comes out clean, about 10 minutes more.
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19
Place the pans on wire racks and let them cool for 10 minutes.
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20
Remove the cupcakes from the pans and cool completely on the racks before frosting, about 40 minutes.
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21
Meanwhile, make the frosting.
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22
Place the butter, peanut butter, and salt in the bowl of a stand mixer fitted with a paddle attachment and beat on high speed until light, fluffy, and fully incorporated, about 3 minutes.
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23
Stop the mixer and scrape down the sides of the bowl and the paddle with a rubber spatula.
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24
Turn the mixer to low and slowly add the powdered sugar until its incorporated, about 30 seconds.
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25
Stop the mixer, scrape down the sides of the bowl and the paddle, and turn the mixer to medium-high speed.
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26
Mix until the frosting is creamy and fluffy, about 3 minutes.
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27
Frost the cupcakes and sprinkle with the chocolate, if using.
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28
If you dont plan to eat the cupcakes within 4 hours, refrigerate them until the frosting stiffens, then tent loosely with plastic wrap for up to 3 days.
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29
Before serving, let the cupcakes sit at room temperature for about 45 minutes to take the chill off.
