For the cake layers
Set the oven to 325° F. Place one rack in the very middle of the oven.
Grease the bottoms of two 8-inch by 2-inch round cake pans with butter, then line the bottoms with a circle of parchment. Then butter and flour the sides of the pans. Set aside
In a small bowl, whisk together the cake flour, baking powder and salt.
To a large bowl, add egg yolks and one-third of the sugar. Mix to break up the yolks and whisk just until the color has lightened a bit.
To the egg yolks, add the whole milk, oil and vanilla. Whisk to combine.
In the bowl of a stand mixer fitted with a whisk attachment, add the egg whites and cream of tartar. Starting on low speed and working your way up to medium-high, beat the egg whites until they begin to look fluffy and opaque. Then, gradually add the remaining sugar, streaming it into the bowl. The egg whites should begin to look glossy. Stop when the mixture is able to hold “medium” or firm peaks, meaning you can lift the whisk straight up out of the bowl, flip it over and the very tip of the meringue will curl over, but not melt into itself.
To the egg yolk/milk/oil mixture, add the dry ingredients. Gently whisk to combine (a few lumps are okay.) You don’t want to work it too much or it will tighten up.
Add about one-third of the egg whites to the large bowl with the rest of the ingredients and, using a flexible rubber spatula, gently fold in the whites.
Repeat this with the rest of the meringue, taking care to try to preserve that air we’ve so painstakingly whipped into the egg whites. (This, along with the baking powder is what will give your cake rise.)
Once fully combined, divide the batter between the prepared pans (it’s worth it to use a scale to make sure the batter is evenly distributed.)
Don’t let the batter sit around — get it right into the preheated oven and set a timer for 1 hour and 10 minutes.
Do not disturb. Bumping the oven or opening the oven door too early can cause your cakes to collapse.
Once the time is up, the cakes should be golden brown, pulling away from the sides of the pan and spring back when gently pressed. Remove from the oven and let them cool in their tins for 5 minutes, then flip them to cool on a wire rack while still in their tins. They should naturally release from the pan, but if they don’t, you can run a small knife or offset spatula along the side to loosen.
Once cooled, slice the cake layers in half using a serrated knife so you end up with four cake layers.
For the whipped cream
In the bowl of a stand mixer fitted with a whisk attachment, beat the powdered sugar, vanilla and mascarpone cheese until smooth and creamy.
Add the heavy whipping cream and cream of tartar.
Start the mixer on low so it doesn’t splash all over you. Gradually increase the speed to medium.
Don’t walk away — whipped cream can curdle in a matter of seconds. You want it to be spreadable but thick enough to hold a stiff peak.
On a cake plate, place one cake layer so the inside is exposed and the “crust” is on the bottom. This will help the coffee soak in.
Using a pastry brush or a squeeze bottle, drench the layer with about one-fourth of the coffee. Don’t be shy, keep going as long as it keeps soaking up the coffee (but save enough for the others!)
Top the layer with one-fourth of the whipped cream. Repeat until for the next three layers.
To finish, lightly dust the top with cocoa powder.
This cake does benefit from about an hour to set and chill in the fridge, but you can serve it immediately or within a day.