Center a rack in the oven, and heat the oven to 400 degrees. Generously butter the sides of an 8-inch springform pan. (You won't be using the base.) Line a baking sheet with baking parchment, and put the springform ring on it.

Melt the chocolate, stirring occasionally, in a large heatproof bowl set over (not in) a pan of simmering water. When it's smooth, whisk in the espresso or coffee. Remove from the heat, and whisk in the butter, tablespoon by tablespoon. Gently whisk in the sugar and one pinch of salt, then add the egg yolks, one at a time. You'll have a lovely, velvety mixture.

Using an electric mixer and a clean, dry bowl, whip the egg whites and the remaining pinch of salt until they are firm but still glossy. Very gingerly whisk about a fourth of the egg whites into the chocolate mixture. Switch to a rubber spatula, and gently fold in the remainder of the whites.

Scrape a generous third of the mixture into the buttered ring on the baking sheet. Cover and refrigerate the remaining mousse.

Bake the mousse in the ring for 15 minutes, at which point it will be puffed. Transfer the baking sheet to a cooling rack, and let the cake cool to room temperature. There will be a dip in the center of it. Place the cake, still on the baking sheet, in the refrigerator.

Now you have a choice:

To serve baked and chilled (recommended): Chill the base of the cake for at least an hour. Heat the oven to 400 degrees. Scrape the chilled mousse onto the chilled base (still on the baking sheet). Bake for 30 minutes or until top is puffed and dry. It will crack, and a knife inserted inside a crack will come out almost dry.

Transfer the baking sheet to a cooling rack, and cool to room temperature. Chill for at least four hours or overnight. Remove the sides of the springform pan. (Run a blunt knife around the edges or warm the pan with a hairdryer.) Carefully transfer the cake to a serving plate, and dust with cocoa. Serve with whipped cream or ice cream if desired.

To serve baked and warm: Bake as for "baked and chilled" (above). After you have transferred the baking sheet, with the cake on it, to a cooling rack, wait 5 minutes, then run a blunt knife around the edges of the cake and remove the sides of the pan. The cake will sink so just let it settle for another 5 minutes. Transfer to a serving platter, and dust with cocoa. Serve with whipped cream or ice cream if desired.

To serve chilled: Let the base of the cake chill thoroughly. Scrape the remaining mousse over the base. Cover and refrigerate at least 6 hours, or overnight.

Transfer the springform pan to a serving plate and remove the ring. (Run a blunt knife around the edges or warm the pan with a hairdryer.) Dust with cocoa. Serve with whipped cream or ice cream if desired.

Serves six.

"Around My French Table" by Dorie Greenspan (2010 Houghton Mifflin Harcourt)