For Sauce: Combine whipping cream, sugar, unsweetened cocoa powder and unsalted butter in heavy medium saucepan. Bring to a simmer over medium - ~low heat, whisking constantly until butter melts and sugar and cocoa dissolve. Remove from heat and whisk in vanilla extract and instant coffee powder. Cool sauce. (Can be prepared 1 week ahead. Cover and refrigerate.)

For Mousse: Line 9x5x3-inch loaf pan with parchment paper. Combine Milky Way pieces, semisweet chocolate, unsweetened chocolate and unsalted butter in large heatproof bowl set over pan of simmering water. Stir constantly until melted (mixture will look grainy). Remove bowl from over water and cool mixture slightly. Combine sugar and water in heavy, small saucepan. Stir over low heat until sugar dissolves. Increase heat to high and boil syrup until temperature reaches 230 degrees on candy thermometer. Meanwhile, beat egg whites and cream of tartar to soft peaks in bowl. Gradually pour boiling syrup into egg whites, beating until soft peaks form and whites are cool, about 5 minutes. Using rubber spatula, fold whites into chocolate mixture in 2 additions. Spoon mousse into prepared pan. Cover with plastic wrap and freeze until firm, at least 6 hours. (Can be prepared 1 week ahead. Keep frozen)

To Assemble: Heat sauce to lukewarm over low heat. Remove plastic wrap from top of mouse. Invert mousse ontol platter. Remove pan and peel off parchment paper. Slice mousse crosswise into 12 slices. Cut each slice diagonally into 2 triangles. For each serving, arrange 2 mousse triangles on plate; spoon warm chocolate sauce over and around mousse and serve.