1. Preheat oven to 350 degrees. Line a 13-by-17-inch rimmed baking sheet with parchment paper. Melt chocolate with coffee or water in a heatproof bowl set over a saucepan of simmering water, stirring until smooth. Let cool completely. Whisk together yolks and granulated sugar with a mixer on medium speed until pale and thick, about 3 minutes. Reduce speed to low, and slowly add chocolate mixture, whisking until incorporated.

2. Whisk whites in a medium bowl until soft peaks form. Gently fold whites into chocolate mixture, using a rubber spatula, until combined, being careful not to deflate whites.

3. Spread batter evenly on lined baking sheet. Bake until cake is cooked through, about 20 minutes. Transfer sheet to a wire rack, and let cool slightly, about 10 minutes.

4. Loosen edges of cake from pan using a knife or an offset spatula. Dust top with cocoa powder, and turn out onto wire rack. Remove parchment, and let cool at least 1 hour. Beat cream and confectioners' sugar until soft peaks form. Spread evenly over top of cake, leaving a 1-inch border around the edges, and tightly roll, starting at 1 short side. Transfer to a baking sheet, and refrigerate for at least 1 hour or overnight. Lightly dust cake with cocoa powder.