Butter and flour an 8-inch cheesecake pan.
In a 3- to 4-quart pan over low heat, frequently stir chocolate, 1 1/2 cups butter, espresso, and 3/4 cup sugar just until chocolate is melted and mixture is smooth, 12 to 15 minutes.
In a bowl, beat eggs to blend. Whisk in chocolate mixture until well blended. Pour into cheesecake pan.
Bake torte in a 350° regular or convection oven until center barely jiggles when pan is gently shaken, about 40 minutes. Let cool on a rack for 30 minutes (center of torte will sink), then chill until cold, at least 1 1/2 hours.
Meanwhile, in a food processor or blender, whirl 1 quart raspberries until smooth. Rub purée through a fine strainer into a bowl; discard seeds. Stir 1/4 cup sugar into raspberry puree; taste, and add more sugar if desired.
Spoon raspberry sauce equally onto plates. Run a knife between torte and pan rim; remove rim. Cut torte into wedges. Arrange a wedge on sauce on each plate. Scatter remaining 1 cup raspberries and marionberries over desserts, and garnish with mint sprigs. Serve with scoops of lemon sherbet.
Note: If you don't have an espresso machine, buy prepared espresso from a coffee shop or use strong brewed coffee: Pour 3/4 cup hot (about 195°) water through 1/3 cup ground coffee in a filter cone. If making torte and sauce up to 1 day ahead, chill separately airtight.