The satiny mocha glaze doubles as a sauce for this cream-filled coffee sponge cake.
Adjust oven rack to top of lower third of oven. Heat to 400°F. Grease 15 x 10-1/2 x 1-inch metal baking pan. Line bottom with parchment or waxed paper; grease paper; flour pan and paper.
Pour boiling water over coffee crystals in bowl; set aside to cool.
Sift flour, baking powder and salt onto parchment or waxed paper 3 times.
Add all but 2 tablespoons of the sugar and the egg yolks to cooled coffee. Beat on medium speed until mixture falls in ribbon when beaters are lifted, 3 to 5 minutes.
Starting at back of bowl, with rubber spatula, cut down through batter, across bottom of bowl and up side, bringing batter over on itself on top; rotate bowl quarter-turn and repeat, until flour is just incorporated.
Using clean beaters and bowl, beat egg whites and cream of tartar until soft peaks form. Add remaining 2 tablespoons sugar in slow stream, beating until stiff peaks form.
Fold one-fifth beaten whites into yolk mixture; stir to lighten. Fold in remaining whites. Scrape into pan.
Bake in 400°F oven 10 minutes or until center springs back when lightly pressed with fingertip.
Loosen cake around edges with small metal spatula. Invert on double-thickness paper toweling dusted with cocoa powder or powdered sugar. Peel off paper. Roll up cake and toweling together, from short end. Cool, seam side down, on rack.
Prepare filling. Unroll cooled cake and towel. Spread cake evenly with filling. Reroll from short end, using towel as guide. Refrigerate while making glaze. Drizzle part of glaze over top of roll. Refrigerate until ready to serve. Garnish. Pass remaining glaze.
Whipped-Cream Filling: Beat heavy cream, powdered sugar and vanilla in bowl until stiff peaks form.
Mocha Glaze: Melt semisweet chocolate pieces, water, unsalted butter, instant coffee crystals and vanilla in small saucepan over very low heat; stir until melted and smooth. Cool. Gently reheat glaze if it becomes too thick to pour.