Special equipment: a 9 1/2 to 10-inch springform pan (about 26 cm)
Beat 2 cups cream with 6 tablespoons sugar in a bowl with an electric mixer at medium speed until it just holds soft peaks, 2 to 4 minutes. Reduce speed to low, then add mascarpone and mix until combined.
Spread 1 1/4 cups mascarpone mixture evenly in bottom of springform pan and cover with 14 wafers, slightly overlapping, if necessary. Spread with another 1 1/4 cups mascarpone mixture, followed by 14 more wafers arranged in the same manner. Spread remaining mascarpone mixture on top. Smooth top with a metal offset spatula, then cover pan with foil and freeze until firm, about 1 hour. Transfer from freezer to refrigerator and chill, covered, until a sharp knife inserted into center cuts through softened wafers easily, about 8 hours.
Pulse remaining chocolate wafers in a food processor until finely ground.
Beat remaining cup cream with espresso powder and remaining 3 tablespoons sugar using electric mixer at medium speed until it just holds stiff peaks, about 3 minutes.
Remove foil and side of pan and frost cake all over with espresso cream. Sprinkle edge of top lightly with wafer crumbs. Serve cold.