Ingredients
1/2 cup unbleached all-purpose flour
1/3 cup sugar
1/4 teaspoon kosher salt (we use Diamond Crystal)
3 tablespoons Dutch-process cocoa powder
3 tablespoons chilled unsalted butter, cut into 1/2-inch cubes
1 large egg yolk
1 tablespoon heavy cream
1 pint coffee ice cream
1 cup chocolate ice cream (1/2 pint)
3/4 cup plus 6 tablespoons sugar
1/4 teaspoon kosher salt (we use Diamond Crystal)
1/3 cup heavy cream
3 large egg whites
1/4 teaspoon cream of tartar
Cocoa nibs or chocolate-covered espresso beans, for serving (optional)
Preparation
Crust: In a food processor, pulse flour, sugar, salt, and cocoa to combine. Add butter; pulse until mixture resembles coarse meal with some pea-size pieces remaining. Stir together yolk and cream; drizzle over flour mixture, pulsing several times to combine. Transfer mixture to a 9-inch fluted tart pan with a removable bottom, pressing evenly into bottom and up sides. Refrigerate until firm, about 20 minutes.
Preheat oven to 350°F. Prick bottom of crust at 1-inch intervals with the tines of a fork. Bake until set and dry, about 15 minutes. Let cool completely on a wire rack.
Meanwhile, temper both ice creams in refrigerator until softened but not melting, about 20 minutes. Stir together ice creams until no streaks remain; transfer to cooled crust, spreading all the way to edges and smoothing top with an offset spatula. Freeze until firm, about 1 hour (or wrap in plastic at this point and freeze up to 1 week).
Caramel Meringue: In a small saucepan, combine 6 tablespoons sugar, salt, and 1 tablespoon water. Cover; cook over medium heat, swirling a few times, until boiling and sugar has dissolved, about 5 minutes. Uncover; continue boiling, undisturbed, until mixture turns medium amber, about 3 minutes more. Remove from heat. Carefully add cream (it will bubble and splatter); stir until smooth. Transfer to a heatproof bowl.
Combine egg whites, remaining 3/4 cup sugar, and cream of tartar in a heatproof mixer bowl set over (but not in) a pot of simmering water. Whisk constantly just until mixture is warm to the touch and sugar has dissolved (it should feel smooth when rubbed between your fingers). Remove from heat; beat on high speed until stiff peaks form and mixture is no longer warm to the touch, 7 to 9 minutes.
Reduce speed to low and beat in a scant 1/4 cup caramel just to combine (do not overmix; caramel will still be warm). Spoon meringue over ice cream and freeze until firm, at least 4 hours (tart can be wrapped in plastic at this point and frozen up to 3 days). Cover and refrigerate remaining caramel until ready to serve.
Remove tart from pan; transfer to a cake plate. Drizzle top with remaining caramel (or serve alongside); sprinkle with cocoa nibs or decorate with chocolate espresso beans. Cut into wedges, dipping knife blade in hot water and wiping clean between each cut to ensure clean slices.
