Preheat the oven to 350°F / 175°C. Generously butter a 7" springform pan and coat it with sugar.
Brew 60g (2 oz.) of espresso. In a medium sized mixing bowl, mix together the cocoa powder and freshly brewed espresso until no lumps remain.
Add in the vegetable oil, sugar, egg, and sour cream. Whisk vigorously until combined.
Add in the flour, baking powder, and salt. Gently fold the dry ingredients into the wet ingredients just until no streaks of flour remain.
Pour the chocolate cake batter into your prepared 7" cake pan. Bake for 28-35 minutes. To check for doneness: I recommend using a thermometer for the best results: when stuck into the middle of the cake the thermometer should read 205°F / 96°C. The cake is also done when the center no longer jiggles and the top becomes more firm.
Allow the cake to rest in the cake pan for at least 15 minutes before transferring it out to a cooling rack to cool to room temperature.
Ladyfinger & Espresso Soak
Once the cake has cooled and you're ready to assemble it, place the cake back into the springform pan. Use a small straw to poke a bunch of holes in to the cake.
Pour 60g (2 oz.) of cooled espresso over the cake.
In a small shallow bowl, combine the cold brew and coffee liquor (optional).
Dip the ladyfingers into the coffee mixture on both sides, and place them on top of the cake. You will need to cut ~two of the ladyfingers to fit.
Cover the cake and let it sit in the fridge for 30 minutes to allow time for the coffee to soften the ladyfingers.
Mascarpone Whipped Cream
In a medium sized mixing bowl with a hand or stand mixer, beat together the mascarpone, sugar, and vanilla just until the mixture is cohesive.
Slowly beat in the heavy whipping cream. Once all of the heavy cream has been added, beat the mixture until stiff peaks form.
Remove the cake from the fridge and pile it high with the mascarpone whipped cream. Take the cake out of the pan, dust it with cocoa powder, slice, serve, and enjoy!
I wouldn't recommend using the toothpick test for this cake. While testing, I found that once the toothpick came out of the cake clean, the cake was over-done and dry after baking. Instead, I highly recommend checking the internal temperature of the cake with a thermometer (as noted in the recipe)