"This is a variation on Dionne Lucas' roulade recipe. I discovered it when searching for a decadent gluten-free dessert for a friend. The cake is versatile and you can experiment with different fillings."
Line the bottom of a 15- by 10- by 1-inch shallow baking pan with a large piece of parchment paper, letting paper hang over ends by 2 inches.
Melt chocolate with water in a small heavy saucepan over very low heat, stirring. Cool to lukewarm.
Beat yolks, 1/3 cup sugar, and salt in a large bowl until thick and pale, about 5 minutes in a standing mixer or about 8 minutes with a hand-held mixer.
Fold in melted chocolate until blended.
Beat whites with cleaned beaters until they hold soft peaks.
Gradually add remaining 1/3 cup sugar and beat until whites just hold stiff peaks.
Fold one third of whites into melted-chocolate mixture to lighten, then fold in remaining whites gently but thoroughly.
Spread batter evenly in baking pan and bake in middle of oven 15 to 18 minutes - until top is dry to the touch,.
Transfer pan to a rack and cover top with 2 layers of damp paper towels and let stand 5 minutes, then remove towels and cool completely. Sift cocoa powder over top of cake layer and overlap 2 layers of wax paper lengthwise over cake.
Place a baking sheet over paper and invert cake onto it, gently peeling off wax paper lining.
Beat cream with confectioners sugar and liqueur until it just holds stiff peaks.
Spread filling evenly over cake.
Using wax paper as an aid, roll up cake jelly roll–style, beginning with the long side. Carefully transfer, seam side down, to a platter, using wax paper to help slide cake.
Dust cake generously with cocoa powder and confectioners sugar.