Set a roasting pan on lowest rack of oven. Fill pan halfway with water. Preheat oven to 350 degrees. Coat a 9-by-4 1/4-inch loaf pan with cooking spray. Cut parchment paper large enough to cover bottom and to hang over long sides of loaf pan. Place in loaf pan; coat with cooking spray.
Put chocolate and butter in a medium heatproof bowl set over a pan of simmering water. Heat, stirring, until melted. Remove from heat; let cool slightly.
Put heavy cream in the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-high speed until soft peaks form, 1 to 2 minutes. Transfer to a small bowl; set aside.
Put eggs and sugar in the clean bowl of an electric mixer fitted with the whisk attachment and whisk on medium-high speed until tripled in volume, 3 to 4 minutes. Gently fold half of the chocolate mixture into egg mixture until almost incorporated; fold in remaining chocolate mixture. Fold in reserved whipped cream and the peppermint extract.
Reduce oven temperature to 325 degrees. Pour batter into prepared loaf pan. Set loaf pan in roasting pan. Bake until chocolate loses its sheen and appears set when loaf pan is jiggled, about 25 minutes.
Carefully transfer roasting pan to a heatproof surface. Remove loaf pan from water bath; let cool completely on a rack.
Pulling up on parchment, remove torte from pan; transfer to a baking sheet. Refrigerate 30 minutes.
Using an offset spatula, gently loosen torte from parchment and then invert onto a serving platter. Torte can be made up to two days in advance.
Using a sieve, lightly dust torte with cocoa. Garnish with fresh and chocolate mint leaves, and whipped cream if desired.
Torte can be made up to two days in advance.