6 ounces gf bittersweet chocolate, coarsely chopped 3 Tablespoons unsalted butter 2 Tablespoons coffee 1 teaspoon gf vanilla flavoring 3 large egg yolks 3 Tablespoons coffee 3 Tablespoons sugar 3 large egg whites, at room temperature 1/4 teaspoon cream of tartar 1/4 cup sugar 1/2 cup cold heavy cream whipped cream 1 ounce gf semisweet or gf bittersweet chocolate, grated for garnish
Heat 1 inch water in a large skillet over low heat until bubbles form along the bottom; adjust the heat to maintain the water at this temperature. Combine 6 ounces of gf chocolate, unsalted butter, 2 tablespoons coffee or water, and gf vanilla in a large heatproof bowl. Set the bowl in the water bath and stir until the chocolate is melted. Remove from the water and set aside.
Whisk egg yolks, 3 tablespoons coffee, and 3 tablespoons sugar together thoroughly in a heatproof bowl. Set the bowl in the water bath and, whisking constantly, heat the mixture until thick and puffy, like marshmallow sauce. Remove from the water bath and whisk thoroughly into the melted chocolate. Let cool to room temperature.
In a separate bowl, beat egg whites on medium speed until foamy. Add the cream of tartar and beat until soft peaks form. Gradually beat in 1/4 cup sugar. Increase the speed to high and beat until the peaks are stiff. Using a large rubber spatula, stir one-quarter of the egg whites into the chocolate mixture to lighten it, then gently fold in the remaining whites.
In another bowl, beat 1/2 cup cold heavy cream on high speed until soft peaks form. Gently but thoroughly fold the cream into the chocolate mixture. Turn mousse 6- to 8-ounce individual cups.
Refrigerate for at least 4 hours or up to 24 hours. Serve with fresh whipped cream. If you wish, sprinkle with 1 ounce grated gf chocolate.