Instructions

For the Cake

1. Preheat the oven to 350 degrees F. Spray the bottom and sides of three 8-inch round cake pans with nonstick spray, line the bottom with parchment paper, and spray again with the nonstick spray. Set aside.
2. In a stand mixer fitted with the paddle attachment, combine the butter, granulated sugar, brown sugar, and red velvet flavor. Beat the ingredients on medium-high speed for about 5 minutes, until the mixture is light in color and slightly increased in volume.
3. Lower the speed and add the eggs, one at a time, mixing until each egg is incorporated before adding the next.
4. In a separate bowl, sift the flour and cocoa powder. Sprinkle the salt on top of the dry ingredients and whisk until combined.
5. With the stand mixer on low speed, alternately add the dry ingredients and the buttermilk into the butter mixture, beginning and ending with dry ingredients. Mix until just incorporated. Fold in the sour cream.
6. In a small bowl, combine the vinegar and baking soda until it fizzes and then add to the batter. Stir until blended.
7. Divide the batter evenly between the prepared pans (about 16 oz. of batter in each).
8. Bake for 22-25 minutes. Mine took 22 minutes.
9. Let pans cool on wire rack for about 10-15 minutes before inverting the cakes onto racks. Let cool completely.
10. Level cakes if needed. Chill the cakes for at least an hour before using. Can be stored the freezer for up to one month wrapped well in plastic wrap and foil. 

FOR THE TOASTED MARSHMALLOW FILLING

1. Place the marshmallows on a cookie sheet lined with parchment paper and sprayed with nonstick cooking spray. Place on the middle rack of the oven, and broil the marshmallows until golden brown on top, about 30-60 seconds. Keep your eye on them!
2. Remove the pan from the oven and gently turn the marshmallows over and broil them again until they are golden brown on the other side.
3. In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and powdered sugar and beat on low until blended. Add the vanilla and mix on medium speed for about three minutes
4. With the mixer on low speed, add the marshmallow cream and (cooled) toasted marshmallows, and mix for about one minute. 

For the Chocolate Frosting

1. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter for about 2 minutes on medium-high speed.
2. Turn the mixer to low speed and slowly add the melted, slightly cooled chocolate.
3. Gradually add the powdered sugar, vanilla and pinch of salt. Add the sour cream.
4. Turn the mixer to medium-high speed and beat the frosting for about 5 minutes.
5. Before frosting the cake, use a wooden spoon to mix the frosting by hand to push out air bubbles and create a smooth and silky texture. 

Assembly

1. On an 8-inch cake board or cake plate, smear a small amount of frosting on it. Place your first cake layer top-side up. Pipe a rim of chocolate frosting around the edge of the cake layer. Inside the chocolate circle, pipe or spread half of the marshmallow filling.
2. Place the second cake layer top-side up and repeat with the frosting and filling. Place the final cake layer bottom-side up. Crumb coat the cake with a thin layer of chocolate frosting. Freeze the cake for about 10-15 minutes to set the crumb coat.
3. Continue to frost and decorate the cake with the remaining chocolate frosting.
4. You can refrigerate this cake, covered, for a day and it will stay moist, or freeze it, covered, for up to a week. Serve at room temperature.