for the champagne buttercream (adapted from Sprinkle Bakes):
1/4 cup Brut champagne or prosecco
1/2 stick unsalted butter, softened
3/4 cup confectioners’ sugar
Instructions
To make the cake: Preheat oven to 350 degrees and line three 4-ounce ramekins with parchment paper circles. Sift together flour, sugar, cocoa powder, baking soda, and salt in a small bowl. In a medium bowl, whisk brown sugar, vanilla extract, vinegar, vegetable oil, and coffee until blended. Pour the dry ingredients into the wet and fold just until incorporated. Divide batter evenly between each ramekin and bake at 350 degrees for 15-18 minutes, or until domes spring back when touched and a toothpick inserted in the center of the cake comes out clean.
Let cakes cool briefly, then run a sharp knife around the edges and invert the ramekins. The layers should slide out easily. Let cool for another 5-10 minutes, or until barely warm to the touch, then wrap well in two layers of plastic wrap and freeze until ready to use. Alternatively, move straight to making the jam and buttercream.
To make the jam: Combine the sugar and champagne in a small saucepot over medium-high heat and stir until the sugar dissolves. Add the cranberries and bring the mixture to a boil, then reduce heat to medium-low and simmer for 5-7 minutes, or until the cranberries have burst and the mixture thickens. Set aside and let cool.
To make the buttercream: Place the champagne in a small saucepot over medium heat. Let simmer for about 5 minutes, or until the champagne reduces to one tablespoon. keep an eye on it to ensure it doesn’t boil dry. When it’s reduced, pour into a small container and let cool completely.
Meanwhile, use an electric mixer or plenty of arm strength to cream the butter and confectioners’ sugar together until thick and fluffy. Add the reduced champagne, a teaspoon at a time, beating after each addition, until the buttercream reaches your desired consistency.
Frost the cake as desired — I used Tessa’s wonderful tutorial here, with an extra layer of buttercream underneath the jam. And enjoy!