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1
Preheat oven to 325 degrees F (165 degrees C). Line 14 muffin cups with paper muffin liners.
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2
Whisk flour, 1 cup sugar, cocoa powder, baking powder, baking soda, and salt together in a large bowl. Add milk, vegetable oil, 1 egg, and vanilla extract; mix using an electric mixer on low speed until just combined. Increase speed to medium and mix well.
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3
Reduce mixer speed to low and slowly pour boiling water into flour mixture. Increase speed to medium and beat for 1 minute.
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4
Fill each muffin cup halfway and place a peanut butter cup in the center of each.
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5
Bake in preheated oven until golden and tops spring back when lightly pressed, about 20 minutes. Remove from oven and cool in pan for 5 to 10 minutes. Cool completely on a cooling rack before frosting.
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6
Set oven rack about 6 inches from the heat source and preheat the oven's broiler.
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7
Whisk egg whites and sugar together in a bowl until just combined. Pour egg mixture into a small saucepan and heat over medium heat until sugar is dissolved. Return egg mixture to bowl. Whisk egg mixture on high speed until still peaks form.
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8
Place peanut butter in a microwave-safe bowl. Heat on high at 15-second intervals, stirring in between, until peanut butter is thin and creamy. Gently fold melted peanut butter into egg whites. Transfer peanut butter frosting to a piping bag.
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9
Pipe frosting onto cooled cupcakes.
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10
Toast cupcakes under preheated broiler until lightly browned, about 1 minute.
