PREHEAT oven to 375° F. Grease 15 1/2 x 10 1/2 x 1-inch jelly-roll pan; line with parchment or wax paper. Grease and flour wax paper. Sprinkle thin, cotton kitchen towel with powdered sugar.
BEAT egg whites in small mixer bowl until foamy. Add cream of tartar and beat until soft peaks form. Gradually beat 1/3 cup granulated sugar until stiff, shiny peaks form. Set aside.
BEAT egg yolks and vanilla extract in large mixer bowl for about 5 minutes or until thick and pale. Gradually beat in remaining 1/3 cup granulated sugar. Fold in flour and baking cocoa.
FOLD about one-third of the whipped egg whites into the chocolate mixture to lighten, then turn the chocolate mixture into the whites and fold it in gently but thoroughly. Spread evenly into prepared pan.
BAKE for 12 minutes or until cake springs back when touched. Immediately turn cake onto prepared towel. Carefully peel off paper. Roll up cake and towel together, starting with narrow end. Cool completely, seam-side-down, on wire rack.
FOR CHOCOLATE CREAM:
BEAT whipping cream, granulated sugar, baking cocoa and liqueur in large mixer bowl until firm. If desired, fold in ground nuts. Carefully unroll cake. Spread cream mixture evenly over cake, reserving 2 tablespoons. Re-roll cake.
CUT a 1-inch-thick diagonal slice from one end of the cake roll. Place cake roll on serving platter, seam-side-down. Use the reserved cream mixture to attach the cut piece against the side of the cake roll to resemble a knot. Cover with plastic wrap and refrigerate until serving time.
AT SERVING TIME, dust the cake with 1/4 cup powdered sugar to resemble snow. Decorate with candy mushrooms, if desired.
Cook's Tip: The filled cake can be kept in the refrigerator overnight or wrapped airtight and frozen for 2 weeks. Keep wrapped and thaw in the refrigerator.