FOR SAUCE: Combine whipping cream, sugar, unsweetened cocoa powder and unsalted butter in heavy medium saucepan. Bring to 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 termperature reaches 230 deg. 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 stiff peaks form and whites are cool, about 5 min.