Closely adapted from Maida Heatter. While I normally eschew appearance over taste, this is a cake that really benefits from a dramatically dark chocolate batter, so if you have any double dark Dutched cocoa powder, I recommend measuring your cocoa powder, removing 1-2 tablespoons, and substituting in the double dark Dutched cocoa. I do not recommend using all double dark Dutched however--I think regular Dutched cocoa powder tastes vastly superior.
Place baking rack in the lower third of the oven. Preheat to 375 F.
Thoroughly spray a 9-inch (10 cup) bundt pan with baking spray (the oil/flour combination). Set aside.
Whisk the cocoa powder into the hot coffee until dissolved and smooth. Set aside to cool.
Whisk together the flour, baking powder and salt. Set aside.
Cream the butter in the bowl of a stand mixer or in a large bowl using a hand mixer, and then add 1 1/4 cups of the sugar. Cream until fluffy, scraping the sides and bottom of the bowl as needed.
Add the egg yolks one at a time, beating after each and scraping the sides and bottom of the bowl as needed. Beat in the vanilla.
With the mixer on low speed, add the flour mixture in three additions and the milk in 2 additions, alternating, and beginning and ending with the flour mixture. Finish mixing by hand to be sure it is all incorporated.
Remove 2 cups of the cake batter and transfer it to a medium-large bowl. This will be your "white" batter.
Add the cocoa-coffee mixture to the remaining cake batter and mix until smooth. Set aside.
Using a clean bowl--either a new bowl for your stand mixer or a bowl for the hand mixer, and being sure to use scrupulously clean beater attachments (any trace of fat in the bowl or on the beaters will cause your egg whites to deflate), add the egg whites and a pinch of fine sea salt to be whipped.
Beat the egg whites until they are foaming on medium speed. Add the pinch of cream of tartar. Continue beating until they are starting to come together. Sprinkle the reserved 1/4 cup sugar over the beaten egg whites.
Turning your beater up to high, beat the egg whites until they hold soft peaks. You want them just shy of being totally stiff or dry.
Using a half-cup measuring cup, add a half cup of whipped egg whites to each of your batters and fold it in. Repeat until all of the egg whites are used up and have been folded into each of the batters.
You will have more chocolate batter than vanilla--this is fine.
The easiest way to spoon this batter is using a tiny cookie dough scoop and the next smallest size of cookie dough scoop. If you do not have cookie dough scoops (you should!), you can use a tablespoon and teaspoon. Place about 5 well rounded tablespoons of the chocolate dough evenly spaced around the bottom of the bundt pan.
Place a rounded teaspoon of the white cake batter in the space between each tablespoon of chocolate dough.
For the next layer, place your round tablespoons of chocolate batter over the blobs of white cake batter, and then place the rounded teaspoons of white batter inbetween those blobs. Repeat until all of the batter is used up.
Place the cake pan level on your counter. Briskly rotate it once each direction, keeping it level. This is what mine looked like. Do not do anything else to swirl the batter.
Bake for around 55-60 minutes, until a cake tester inserted into the center of the cake comes out clean or with only a few crumbs attached.
Cool the cake in the pan for 15 minutes, and then turn the cake out onto a parchment paper lined cooling rack (the parchment paper helps keep the cake from sinking into the cooling rack).
Cool completely before sprinkling with powdered sugar if desired and slicing to serve.