How to Make It
Preheat the oven to 350°. Generously coat the cups of three 12-cup muffin tins with nonstick spray. In a small bowl, whisk the buttermilk with the coffee and vanilla. In a medium bowl, sift the flour with the cocoa powder, baking soda and salt.
In a large bowl, using a hand mixer, beat the granulated sugar with the oil at medium-high speed until blended; beat in the egg. At low speed, beat in the dry ingredients and the buttermilk mixture in 3 alternating batches. Spoon about 2 tablespoons of batter into each muffin cup.
Bake the cakes for about 12 minutes, until risen and a toothpick inserted in the centers comes out clean. Let cool in the pans for 3 to 5 minutes. Carefully invert the cakes onto a parchment paper–lined baking sheet (this will help them flatten slightly). Let cool completely, about 30 minutes.
In a large bowl, beat the mascarpone, cream, confectioners’ sugar, vanilla, espresso and salt at medium speed until smooth and thick.
Spread the mascarpone filling onto the flat sides of half the cakes. Top with the remaining 18 cakes, pressing down to spread the filling. Dip one half of each cake in the melted chocolate and coat with sprinkles. Refrigerate until set, at least 1 hour or up to 3 days. Serve the cakes cold.