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1
Preheat oven to 350F.
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2
Brush jumbo muffin tins with butter; dust with cocoa powder, tapping out excess.
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3
Whisk together flour, cocoa, baking soda, baking powder, and salt.
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4
With an electric mixer on medium-high speed, cream butter and sugar until pale and fluffy.
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5
Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed.
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6
With mixer on low speed, add flour mixture in two batches, alternating with the sour cream, and mixing until just incorporated after each.
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7
Divide batter evenly among prepared cups, filling each with about 1/2 cup.
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8
Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, about 25 minutes.
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9
Transfer tins to wire racks to cool 5 minutes.
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10
Run a small offset spatula or knife around the edges to loosen; turn out cupcakes onto racks and let cool completely.
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11
Transfer filling to a large heavy-duty resealable plastic bag, and twist bag to remove excess air before sealing.
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12
Snip off one corner of the bag to make a 1/4-inch opening.
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13
(Alternatively, use a pastry bag fitted with a medium round tip, such as #8.)
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14
Using a small melon baller, scoop out center of each cupcake from the bottom, and reserve (to reseal cupcake after filling).
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15
Hollow out each cupcake a bit more, discarding crumbs.
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16
Insert snipped corner of bag (or pastry tip) into each cavity, and squeeze just to fill; replace reserved cake pieces.
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17
Decorate tops with remaining filling.
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18
Filled cupcakes can be stored up to 2 days at room temperature in airtight containers.
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19
Whisk marshmallow cream and butter until smooth.
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20
Cover with plastic wrap, and chill until slightly firm, 15 to 30 minutes, before using.
