Marshmallow-Stuffed Hot Chocolate Cake

Preparation

1. Make the cake

   Step 1

   Adjust an oven rack to the middle position and preheat the oven to 350ºF. Grease a tube pan (without a removable bottom, see note) and line the bottom with parchment paper, with a hole cut out in the middle for the tube. 

   Step 2

   In a medium bowl or liquid measuring cup, whisk together the sour cream, milk, oil, eggs, and vanilla.

   Step 3

   In the bowl of a stand mixer fitted with a paddle, whisk together the flour, granulated and brown sugars, cocoa powder, baking soda, baking powder, and salt by hand.

   Step 4

   With the mixer running on low speed, slowly add the milk mixture. Increase the speed to medium and beat until combined, 20 to 30 seconds. Slowly pour the hot coffee into the batter and mix on low speed until just combined. Using a spatula, give the batter a couple of turns to make sure it is fully mixed.

   Step 5

   Pour the batter evenly into the prepared pan and bake for 25 to 35 minutes, until a wooden skewer or toothpick comes out with the tiniest bit of crumbs.

   Step 6

   Transfer the cake to a wire rack and let cool in the pan for 1 hour. Turn the cake out onto the rack, remove the parchment paper, and let cool completely. Line a sheet pan with parchment paper. Wrap the cake in plastic wrap, place it on the prepared sheet pan, and chill the cake for at least 2 hours and up to overnight.

2. Make the filling

   Step 7

   In the bowl of a stand mixer fitted with a paddle, beat the butter and salt on medium speed until creamy, about 1 minute. Add the confectioners’ sugar and beat again until smooth and creamy, 3 to 4 minutes. Scrape down the sides and add the marshmallow fluff, beating on low speed until well blended. Add the heavy cream and beat again on low speed until combined. Transfer the filling to a bowl and cover. Chill in the refrigerator for at least 2 hours and up to overnight.

3. Fill the cake

   Step 8

   Set the cake rounded-side up on the sheet pan. Using a small paring knife, make a tunnel in the cake by cutting out small curved rectangles (about 2 in long by 1 in wide) into the top of the cake following the curve of the entire cake, directly in the center. Save the pieces of cut-out cake. The tunnel should go down a little more than half the depth of the cake.

   Step 9

   Once you go around the entire cake, spoon the chilled marshmallow filling into the tunnel, filling it halfway (you may have some leftover fluff). Cut off half of the inside of each piece of cut-out rectangle. Replace the cake pieces in the tunnel over the fluff, covering all of it. Turn the cake over onto a serving plate so the cut-out side is on the bottom. Cover the cake loosely with plastic wrap and chill for 1 hour.

4. Make the ganache

   Step 10

   Place the chocolate in a small bowl. Heat the heavy cream in a small saucepan until it is simmering and just about to boil. Pour the cream over the chocolate, cover the bowl with plastic wrap, and let sit for 5 minutes. Remove the plastic wrap and use a butter knife to stir the chocolate into the cream until it is completely smooth. Let the mixture cool to almost room temperature. Once cool and ready to use, stir the ganache a few times before using.

5. Assemble the cake

   Step 11

   Remove the chilled cake from the fridge and pour the ganache over the top, letting it drip down the sides, if desired. Let the ganache set until just tacky, then add the toasted marshmallows to the top. Serve cold or at room temperature. This cake can be stored covered (without the marshmallows) in the refrigerator for 24 hours.