Preheat oven to 200 degrees F (95 degrees C). Grease a 12-cup muffin tin or line cups with paper liners.
Sift cocoa powder into a large bowl and add 5 tablespoons boiling water. Stir into a thick paste, adding more water, 1 tablespoon at a time, if needed. Add 3/4 cup butter and 3/4 cup plus 2 tablespoons sugar and beat with an electric mixer until smooth and creamy. Beat in eggs one at a time, beating well after each addition, until batter is smooth.
Mix flour and baking powder in a bowl and stir into the batter until well combined. Spoon batter into the prepared muffin cups, filling each 2/3 to the top using an ice cream scoop.
Bake in the preheated oven until tops spring back when lightly pressed and a toothpick comes out clean, 10 to 15 minutes. Allow to cool in muffin tin for a few minutes; transfer to wire rack and cool completely, about 1 hour.
Beat confectioners' sugar, peanut butter, 1/4 cup butter, and vanilla extract in a bowl until a thick dough forms. Refrigerate for 30 minutes.
Line a baking sheet with baking parchment.
Remove the peanut butter mixture from the fridge and roll into small balls to make monster eyes. Chill mixture or moisten hands if it starts to stick. Arrange balls on the prepared baking sheet and freeze for 30 minutes.
Place white chocolate in top of a double boiler over simmering water. Stir continuously, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes.
Remove the eyes from the freezer and individually pick them up with a toothpick. Dip balls into melted chocolate leaving a small circle on one side blank. Twirl the toothpick to remove excess chocolate and put back on the parchment lined baking sheet.
Dig out a bit of the peanut butter mixture from each ball where it isn't covered with chocolate to make room for the red decorating gel. Fill the holes with red decorating gel.
Frost cupcakes with chocolate frosting. Place 2 peanut butter eyes on top. Draw on a mouth with the white decorating gel.