TO MAKE CAKE: Preheat the oven to 350 degrees F. With a pastry brush or paper towel, coat an 11x17 inch jelly-roll pan with the butter. This will seem like a lot; be very generous. Sprinkle the flour or cocoa over the butter and shake the pan to coat the butter fully. Tap the edge of the pan on a table to knock out the excess flour. Melt the chocolate in a double boiler or a bowl over a pan of simmering water. Set aside to cool to lukewarm. Beat the egg whites with a pinch of salt until the whites cling to the beater. Add about 1/4 cup of the sugar and beat until the whites form stiff, unwavering peaks. Cream the butter and the other (about) 1/4 cup of the sugar until the mixture is light and fluffy. Add the melted chocolate and beat in the egg yolks one at a time. With a rubber spatula, stir about 1/3 of the beaten egg whites into the chocolate mixture, then pour the chocolate mixture over the rest of the whites. Sprinkle the flour lightly on top. Gently fold the mixture together until no white streaks remain. Pour the batter into the prepared pan, spreading it evenly with a rubber spatula. Bake in the middle of the oven for 15 to 18 minutes, or until the cake shrinks slightly away from the sides of the pan and a knife inserted in the middle comes out clean. It will still be very flat. Loosen the cake from the pan with a sharp knife around the sides and turn it out onto a rack to cool. (Put the rack over the pan and flip the whole thing over to keep the cake from breaking.) TO MAKE FILLING: In a small, heavy saucepan, combine the cream and chocolate and stir over medium heat until the chocolate dissolves. Then reduce the heat to very low and simmer, stirring constantly, until the mixture thickens to the consistency of a pudding. Pour it into a bowl and refrigerate for at least 1 hour. When the mixture is very cold, pour in the rum and vanilla and beat with a whisk or beater until the filling is smooth and creamy and forms soft peaks when the beater is lifted. Do not over-beat or you will get butter. (If this should happen, don't despair; chocolate buttercream makes a perfectly fine filling.) Cut the cake in half to make two layers, each about 8 1/2 inches wide. Spread the filling over one layer and set the other layer on top. Smooth out the edges with a spatula. If one of the cake layers should break, use it on the bottom. Refrigerate on a rack for about 1 hour. : Continued in Part 2 : Copyright (C) 1986 USENET Community Trust