In a bowl of an electric mixer combine the mint leaves, the sugar, the cocoa powder, and the cream, beat the mixture until it holds soft peaks, and spread about 1 mounded teaspoon of it on one side of each wafer. On a platter sandwich the wafers together on their sides to form a log and ice the log with the remaining cream mixture, covering it completely. Chill the cake, covered loosely with plastic wrap, for at least 6 hours or overnight. Cut the cake diagonally into 3/4-inch-thick slices and serve it garnished with the mint sprigs. Gourmet July 1992