Candied flowers such as violets,for garnish (optional)
Preheat the oven to 350°. Lightly butter a 10-by-15-inch jelly-roll pan and line it with wax paper. Butter the paper. In a medium saucepan, melt the chocolate in the coffee over low heat, stirring constantly. Let cool slightly.
In a large bowl, using an electric mixer, beat the egg yolks with the sugar until pale and thick. Add the melted chocolate and beat at low speed until blended.
In a large stainless-steel bowl, using clean beaters, beat the egg whites until firm peaks form. Stir one-fourth of the beaten egg whites into the chocolate mixture to lighten it, then gently fold in the remaining whites.
Spread the batter evenly in the prepared pan and bake for about 15 minutes, or until a toothpick inserted in the center comes out clean. Transfer the pan to a rack. Cover the cake with a kitchen towel and let cool completely. Refrigerate for 30 minutes.
Remove the towel and generously sift cocoa over the entire surface of the cake. Run a knife along the edge of the pan to loosen the cake. Cover the cake with a clean kitchen towel. Place a large flat baking sheet over the towel; invert the cake onto the baking sheet and remove the jelly-roll pan. Gently peel the wax paper off the cake.
Beat the cream until stiff peaks form, then stir in the rum. Spread the whipped cream evenly over the cake. Beginning at a long edge and using the kitchen towel to help guide you, carefully roll up the cake. Transfer the cake to a platter and sift additional cocoa on top. Garnish with candied flowers, if using, and serve.