Unsweetened chocolate, bittersweet chocolate, and instant espresso powder give these cupcakes a deep, rich flavor that is complemented perfectly by the dulce de leche frosting. Dulche de leche is a caramel flavor widely used in Latin American cuisine. Both the cupcakes and the frosting can be made a couple of days in advance and assembled shortly before serving.
Adapted from "Intensely Chocolate" by Carole Bloom
For the cupcakes:
Center a rack in the oven and preheat the oven to 350 degrees. Place the walnuts in a cake or pie pan and toast until lightly golden, about 8 minutes. Remove the pan from the oven and cool on a rack, then finely chop the walnuts. Position the oven racks in the upper and lower thirds and line the cavities of two muffin pans with cupcake papers, or place 24 silicone muffin cups on two baking sheets.
Melt the unsweetened chocolate, bittersweet chocolate, and butter together in the top of a double boiler over low heat, stirring often with a rubber spatula to ensure even melting. Or melt the chocolates and butter in a microwave-safe bowl on low power in 30-second bursts, stirring after each burst. Remove the top pan of the double boiler, if using, and wipe the bottom and sides very dry.
Add the instant espresso powder and vanilla and chocolate extracts, and stir to blend thoroughly.
Whipthe eggs in the bowl of an electric stand mixer using the wire whip attachment or in a large mixing bowl using a handheld mixer on medium speed until they are frothy. Add the superfine sugar and brownsugar and whip together until the mixture is very thick and pale yellow and holds a slowly dissolving ribbon when the beater is lifted, about 5 minutes. Add the melted chocolate mixture and blend completely on low speed.
Over a bowl, sift the flour, then add the salt and toss to blend. Add this mixture to the chocolate mixture in 2 stages, blending well after each addition. Stop and scrape down the sides and bottom of the bowl with a rubber spatula.
Add the walnuts and stir to blend thoroughly. Use a 2-inch-diameter ice cream scoop to divide the batter evenly among the cavities of the cupcake pans.
Bake the cupcakes for 18 minutes, until a cake tester or toothpick inserted in the center of a cupcake comes out slightly moist.
Remove the pans from the oven and cool on a rack for 15 minutes. Lift the cupcakes from the pan and cool completely on racks.
For the dulce de leche frosting:
Beat the butter in the bowl of an electric stand mixer using the fl at beater attachment or in a large mixing bowl using a handheld mixer until soft and fluffy, about 2 minutes. Add the dulce de leche and blend together thoroughly.
Fit a 14-inch pastry bag with an open star tip and fill partway with the frosting. Pipe a large rosette in the center of each cupcake. Or use a small offset metal spatula to spread the top of each cupcake with the frosting. If the frosting is soft, chill the cupcakes for 15 minutes to set the frosting. Serve the cupcakes at room temperature.