Mix 1 ½ cups graham cracker crumbs with 6 tablespoons unsalted butter, melted, until the texture resembles wet sand. Press the mixture into the bottom of a 9-inch springform pan to form an even base.
Bake for 8-10 minutes. Remove and cool while preparing the filling. Leave the oven turned on.
Cheesecake layer:
In a large bowl, beat 1 pound cream cheese, softened, until smooth.
Add ½ cup granulated sugar and mix until fully incorporated. If using an electric mixer, do so until this step.
Add 2 eggs one at a time, mixing well after each addition with a whisk or spatula instead of beating.
Add 1 tablespoon sour cream and 1 teaspoon vanilla extract and mix to incorporate.
Pour the cheesecake batter over the cooled crust.
Bake for 35 to 40 minutes, until the edges are set but the center still jiggles. Turn off the oven, leave the door ajar (slightly open), and let the cheesecake cool inside for 30 minutes.
Remove, cool completely on a wire rack, then refrigerate for at least 2 hours.
Mascarpone cream:
In a bowl, beat 16 ounces mascarpone cheese, cold, until smooth and lump-free. Add 1 teaspoon espresso powder and ¼ cup powdered sugar, and mix in until it's a smooth, even-colored mixture.
In a medium bowl, whip 1 ¼ cups heavy cream, cold, until medium-firm peaks.
Gently fold the whipped cream into the mascarpone mixture with a spatula until incorporated.
If using a piping bag, fill it now and refrigerate it. Or refrigerate the bowl with the cream if spooning it.
Assembly:
Once the cheesecake is fully cold, continue with assembly.
Combine ½ cup prepared espresso coffee and 1 tablespoon coffee liqueur, if using, in a shallow bowl that fits a ladyfinger.
Dip 8 ladyfinger biscuits briefly (a second or two per side, or they might break) into the coffee soak, one at a time. Let excess drip. Arrange them over the chilled cheesecake layer, chopping some as needed to fit the round pan.
Spread the mascarpone cream evenly over the ladyfingers using a spoon or piping bag.
Refrigerate for at least 3 hours, or overnight, to fully set.
Dust with unsweetened cocoa powder just before serving. Keep leftovers chilled and covered to prevent drying. Some of the cocoa might seep into the cream.
Notes
Cream: You can also use heavy whipping cream instead of heavy cream, but I find that it's not as consistent across different brands, so it's not my first recommendation.If you don't have espresso powder, instant coffee also works.