In a bowl, with a mixer on medium speed, beat butter and sugar until smooth. Add eggs, one at a time, beating well after each addition and scraping down sides of bowl as needed.
In another bowl, mix flour, cocoa, baking powder, and salt. Stir in half the flour mixture into butter mixture. Stir in milk just until blended. Add remaining flour mixture and stir just until incorporated. Spoon batter equally into 16 muffin cups (1/3-cup capacity; cups should be almost full) lined with paper baking cups.
Bake in a 350º regular or convection oven until tops spring back when lightly pressed in the center or a wooden skewer inserted into the center comes out clean, about 20 minutes. Let cool in pans on racks for 5 minutes; remove cupcakes from pans and set on racks to cool completely, at least 30 minutes.
With a small, sharp knife, cut a cylinder about 3/4 inch wide and 1 inch deep from the center of the top of each cupcake. Trim off and discard about 1/2 inch from bottom of each cylinder. With a knife, hollow out a small cavity inside each cupcake.
Spoon marshmallow cream into pastry bag fitted with a 1/2-inch plain tip. Twist end of bag tightly to secure. Place tip in cavity of one cupcake; without moving tip, squeeze filling into cavity, to 1/4 inch from top. Repeat to fill remaining cupcakes. Insert cake cylinders into holes.
Spoon chocolate cream cheese frosting into another pastry bag, fitted with a 3/4-inch star tip, and pipe onto tops of cupcakes, or spread on cupcakes with knife.
I thought this too much trouble so I shortened a step. I placed a third of the batter into each cupcake pan, added one miniature marshmallow, then covered that with the rest of the batter. It baked up beautifully and the marshmallow was melted. I iced with the frosting and the cupcakes were gobbled up!
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