Preheat oven to 350°F (180°C).
In the top of a double boiler, combine bittersweet chocolate and unsalted butter. Cook over simmering water, stirring occasionally, until melted and smooth. Remove from heat, and whisk in light brown sugar and granulated sugar until well combined. Let cool slightly, 1 to 2 minutes.
In a medium bowl, whisk together all-purpose flour, unsweetened cocoa powder, kosher salt, and instant espresso powder.
Gradually add eggs to chocolate mixture, whisking until combined. Whisk in vanilla extract. Fold in flour mixture just until combined. Spread batter into cold Chocolate Chip Cookie Crust.
Bake until a wooden pick inserted in center comes out with a few moist crumbs, 55 minutes to 1 hour and 5 minutes. Let cool completely on a wire rack.
Serve with sweetened whipped cream. Store in an airtight container for up to 3 days.
