In a microwave-safe bowl, add the flour, sugar, cocoa powder, baking powder, and salt. Whisk to combine.
1/2 cup all-purpose flour, 1/2 cup granulated sugar, 1/4 cup unsweetened cocoa powder, 1 teaspoon baking powder, Pinch of salt
Add the butter, eggs, and milk. Stir just until smooth.
1/3 cup unsalted butter, 2 large eggs, 1/3 cup whole milk
Microwave on high for one minute or until a toothpick can be inserted into the center and comes out clean. NOTE: Depending on the microwave, it may take up to 90 seconds. Be careful not to overcook, or the cake will become dry and rubbery!
Cut a small piece of the cake from the center, and set aside.
Spoon the marshmallow fluff into the hole, and place the cake piece back on top.
1/4 cup marshmallow fluff + more or less to taste
Serve right away.