Preheat your oven to 350°F. In a bowl, mix together the graham cracker crumbs, light brown sugar, and unsweetened cocoa powder with melted butter. Press the mixture evenly into the bottom of a 9-inch springform pan. Bake the crust for 8 minutes and then set aside.

In a large bowl, beat the cream cheese, mascarpone, granulated sugar, salt, and vanilla extract until smooth. Add the eggs one at a time—mixing gently after each addition—to avoid too much air. Stir in the heavy cream, sour cream, and sifted cornstarch until just combined.

Dip each ladyfinger briefly in the cooled coffee and arrange them in a layer over the baked crust, breaking them apart to fill gaps if needed. Pour the cheesecake filling over the ladyfingers and smooth the surface gently.

Remove the cheesecake from the oven and let it cool at room temperature for 15 minutes, then transfer to a wire rack and chill in the refrigerator for at least 8 hours.

For the topping, whip the chilled heavy whipping cream with powdered sugar until stiff peaks form. Spread or pipe the whipped cream over the chilled cheesecake and dust lightly with cocoa powder before serving.