Preheat oven to 425°F. Line 2 large baking sheets with parchment paper.
Make cakes: Sift flour, unsweetened cocoa, baking soda and salt together into a bowl. Cream butter and sugar in a large bowl of an electric mixer at high speed until fluffy, about 5 minutes. Add egg and beat at medium speed until fully incorporated. Change mixer speed to low and add dry ingredients, alternating with milk, mixing until just blended.
Drop batter 1/2 Tbsp. at a time and about 2 inches apart onto each sheet to make 18 cakes per baking sheet. Bake for 7 minutes, until mini cakes are springy to touch. Let cool on pan for 5 minutes, and then transfer to racks to cool completely. Change parchment paper and repeat to make 72 cakes.
Match cakes in pairs so tops and bottoms are similar in shape. Spread bottoms with cream filling and cover with top.
For Cream Filling, combine 1 stick of unsalted butter and 1 2/3 cups confectioners’ sugar in a bowl and beat at low speed until blended. Then beat at high speed until mixture is fluffy, about 5 minutes. Add 1/2 tsp. pure vanilla extract and 2 cups marshmallow cream; mix at low speed until blended well. (Makes 2 1/3 cups)