Preheat oven to 350°F. Butter or oil a 10-by-15-inch shallow baking pan or jellyroll pan. Line the bottom lengthwise with a piece of waxed or parchment paper that extends up the short sides one inch.

Melt chocolate with water or coffee in a small saucepan over very low heat until it is 75 percent melted. Remove from heat and stir until the remaining chocolate is smooth. Set aside to cool slightly.

Beat egg yolks with an electric mixer until pale and creamy. Add sugar gradually, and continue to beat until yolks are pale and ribbony. Gently stir the chocolate into the yolk mixture.

In a clean bowl with clean beaters, beat egg whites with salt until they hold stiff peaks.

Stir ¼ of egg white mixture into the chocolate-yolk mixture to lighten it.

Fold the remaining whites into the cake batter in three additions.

Pour batter into prepared pan and smooth top. Bake in preheated oven for 15 minutes, or until cake layer feels dry (but very soft) to the touch and a toothpick inserted into the center comes out clean. It will still seem a little underbaked.

Transfer to a cooling rack and cover the top with a light damp towel or two layers of damp paper towels for 10 minutes.

Gently remove towels. Run a knife around the edges of the cake. Sift one tablespoon cocoa over the top of the cake and cover the cake with a layer or waxed or parchment paper that is a little longer than the pan.

Place the back of a baking sheet or a large flat tray over the waxed paper and invert the cake and paper onto it. Gently peel back the parchment or waxed paper that was on the underside of the cake. Sift the remaining tablespoon of cocoa powder over the underside of the cake and cover the layer with a thin tea or flour sack towel.

Using the waxed or parchment paper underneath to help lift and roll the cake, roll the cake from short end to short end with the towel inside. Let cool completely.

Once cool, beat heavy cream with powdered sugar and vanilla until it holds stiff peaks.

Get your serving plate ready and place it near your cake roll.

Gently unroll chocolate cake and remove tea towel. Spread whipped cream filling evenly over cake. Gently use waxed or parchment paper once again to r-eroll cake. Place on serving platter, seam side down.

Serve immediately in 1-inch slices or refrigerate until needed.