Preheat the oven to 350°F / 175°C. Butter a 7" springform pan*, then coat the inside with sugar.
To a medium sized mixing bowl, add the flour, baking powder, salt, sugar, and cubed butter. Use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture together until the butter has completely combined with the dry ingredients and no lumps remain.
Add in the oil, vanilla, sour cream and milk. Whisk the ingredients together just until a cohesive batter has formed.
In a separate bowl, beat the egg whites until they reach stiff peaks.
Add the egg whites to the batter and gently fold them into the batter until the mixture is cohesive.
Pour the cake batter into your prepared pan. Bake for 35-45 minutes, until a toothpick inserted in the center of the cake comes out with only a few moist crumbs.
Allow the cake to rest in the cake pan for at least 15 minutes before transferring it out to a cooling rack.
Espresso Soak
In a small bowl, mix together the freshly brewed espresso, coffee liquor and sugar. Place the mixture in the freezer for ~5 minutes, until it cools to room temperature.
Once the cake has completely cooled to room temperature, slice it in half horizontally to get two even pieces.
Use ~⅓ c. of the mixture to brush on the inside of both halves of the cake.
Place the bottom half of the cake back into the springform pan.
Tiramisu
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.
Spread ¼ of the mascarpone cream on top of the cake in the springform pan.
Quickly dip the ladyfingers into the espresso mixture on both sides, then layer them into a single layer on top of the mascarpone cream.
Spread ⅓ of the remaining mascarpone cream over the ladyfingers.
Place the top half of the cake on to the mascarpone cream - pushing down slightly to secure it.
Cover the cake with plastic wrap and leave it in the fridge to set for at least 30 minutes. Place the remaining mascarpone cream in the fridge as well until you are ready to serve the cake.
Once you're ready to serve the cake, spread the remaining mascarpone cream on top and dust it with some cocoa powder. Then, slice, serve, and enjoy!