Preheat the oven to 400 degrees F (200 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 1/2 cup butter together in a bowl with an electric mixer until light and fluffy. Add 2/3 of the confectioners' sugar, vanilla extract, and salt. Beat well. Add milk, 1 tablespoon at a time, alternating with the remaining 1/3 of the confectioner's sugar until desired consistency is reached.
Frost each cupcake with an even layer of frosting using a spatula or a palette knife.
Use a skewer to poke 2 holes in the top end of each marshmallow half for the ears. Poke 2 holes at the bottom of the front side for each of the nostrils.
Cut the black licorice laces into short lengths: 24 small pieces for the nostrils and thirty-six 1/3-inch lengths for the ears and tails. Stick the shorter licorice pieces into the marshmallows where you've poked holes for the nostrils. Stick 24 of the longer pieces into the marshmallows where you've poked holes for the ears. Attach 1 marshmallow head to each cupcake.
Cover the cupcakes with mini marshmallows, sticking them to the frosting flat-side down. Leave space on the opposite side of the head to attach the tail. Poke a hole there using a skewer, then stick in the remaining licorice pieces for the tails.
Spread the backs of two candy eyeballs with a bit of frosting. Attach to the marshmallow, positioning above the nose. Repeat with remaining eyeballs.