1 cup (2 sticks) unsalted butter, room temperature, plus more for muffin tins
3/4 cup unsweetened cocoa powder, plus more for muffin tins
2 cups all-purpose flour (spooned and leveled)
2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon salt
2 cups sugar
3 large eggs, room temperature
1 cup sour cream, room temperature
FOR THE FILLING
1 1/2 cups marshmallow creme (7.5-ounce jar)
1/2 cup (1 stick) unsalted butter, room temperature
How to make it
Preheat oven to 350. Butter two 6-cup (each with a 1-cup capacity) jumbo muffin pans; dust with cocoa powder to coat, tapping out excess. In a medium bowl, whisk together cocoa, flour, baking powder, soda, and salt.
Using an electric mixer, beat butter and sugar until light. Add eggs one at a time, beating well after each addition. On low speed, add half the flour mixture, followed by sour cream, ending with remaining flour mixture; mix just until incorporated (do not overmix).
Divide batter among prepared muffin cups. Bake until a toothpick inserted in center of a cupcake comes out clean, 25 to 30 minutes, rotating pans halfway through baking. Cool in pans, 5 minutes; remove cupcakes and cool, right side up, on a wire rack.
Meanwhile, prepare filling: In a medium bowl, whisk marshmallow creme and butter until smooth. Chill until slightly firm, 15 to 30 minutes. Transfer mixture to a heavy-duty resealable plastic bag, and seal; cut off one corner of the bag to make a 1/8-inch opening.
Using a small melon baller, scoop out center of each cupcake from the bottom, and reserve (you will use this to plug cupcake after filling). Hollow out each cupcake a bit more, discarding crumbs. Insert tip of plastic bag into each cavity, and squeeze to fill; replace plugs. Using remaining filling in plastic bag, decorate top of cupcakes.