Preheat oven to 350 degrees F & lightly butter a 15"x8" baking pan or sheet. Line baking sheet with parchment paper or wax paper, then butter that well & dust lightly with 2 tablespoons of powdered sugar.
In a mixing bowl, beat egg yolks & 1/4 cup granulated sugar until thick & smooth.
Stir in flour, cocoa & vanilla extract, blending well.
In another bowl, whip egg whites until stiff, but not dry, then fold them into the yolk/flour mixture.
Pour batter into prepared baking sheet & spread out evenly.
Bake 15 minutes, then cool for 5 minutes before turning onto a cloth to cool.
After another 10 minutes peel off the paper.
Combine whipped cream, rum & superfine sugar, & adjust to taste.
Spread this cream mixture over the cake & then roll the cake like a jellyroll, starting with the long edge.
Sprinkle with remaining powdered sugar, before slicing into 3-inch long slices & serving.