Recipe(tried): Brazo Gitano ("Gypsy's Arm" Rolled Sponge Cake) for ELMA (Puerto Rican dessert)

BRAZO GITANO
("Gypsy's Arm" Rolled Sponge Cake)
This is a classic Spanish chocolate cake-rum roll. Here in Puerto Rico my favorite use a vanilla cake instead of the chocolate one.

5 eggs, separated
1/4 C. sugar
1/4 C. flour
1/4 C. cocoa powder
1 tsp. vanilla extract
4 Tbsp. sifted confectioners' sugar
1 C. heavy cream, whipped
1 1/4 tsp. rum extract
1 Tbsp. superfine sugar*

Preheat oven to 350F. Beat egg yolks and 1/4 cup sugar until thick and smooth. Stir in flour, cocoa and vanilla extract. Blend well. Whip egg whites until stiff, but not dry. Fold into yolk and flour mixture. Butter a 15 x 8-inch baking pan. Line with baking parchment paper or wax paper. Butter well again, and dust lightly with 2 tablespoons of the confectioners' sugar. Pour batter into the pan and spread out evenly. Bake for 15 minutes. Cool for 5 minutes, then turn onto a cloth to cool. After 10 minutes peel paper off. Combine whipped cream, rum and sugar. Adjust to taste. Spread over cake and roll cake like a jellyroll, starting with the long edge. Sprinkle with remaining confectioners' sugar. Serve in slices.* Superfine Sugar: Put desired amount of granulated sugar in a blender and blend at high speed for about 30 seconds.