A tres leches cake is a sponge cake with three different kinds of milk poured over top that get absorbed (hence the name sponge). This version gets spooked up thanks to the addition of black cocoa, which gets its darker color from being heavily Dutched (neutralized to be less acidic). Some think it is a little milder in chocolate flavor than natural cocoa powder. It is used here because it makes the cake stark black without relying on food coloring. Topped with ghosts made from a simple Swiss meringue, this cake is perfect for a Halloween celebration.
Preheat the oven to 350 degrees F. Spray an 8-inch round cake pan with nonstick cooking spray.
For the cake: Sift the flour, black cocoa, baking powder and salt into a medium bowl. Combine the eggs, sugar and vanilla in the bowl of a stand mixer fitted with the paddle attachment and beat on medium-high speed until very light yellow and fluffy, about 10 minutes (really!). Reduce the speed to low and slowly add half of the flour mixture, then all the buttermilk and finally the remaining flour mixture, scraping down the sides of the bowl as needed. Pour the batter into the prepared pan, smooth the top and bake until the cake springs back when touched lightly and a toothpick or cake tester comes out clean, about 45 minutes. Let cool in the pan for 30 minutes. Keep the cake in the pan and place on a plate that's larger than the pan.
Whisk the heavy cream, sweetened condensed milk and evaporated milk in a large liquid measuring cup or medium bowl until smooth. Poke a lot of holes all over the cake using a wooden skewer. Slowly pour 1/3 of the milk mixture over the cake, letting most of it get absorbed before pouring more. Repeat this step twice more until the milk mixture is gone. Cover the pan and refrigerate until the milk is completely soaked in (you will see some residual milk on the top and a little on the sides), about 6 hours, up to overnight.
Invert the cake onto a large rimmed plate that can hold the excess soaking liquid. Pour the excess liquid into a container with a spout and set aside. Carefully invert the cake, domed-side up, onto a cake stand or platter. Drizzle with the reserved soaking liquid. Decorate the platter with Halloween sprinkles, if desired.
Make the Swiss meringue: Just before serving, bring a few inches of water to a boil in a saucepan that can hold a heatproof stand mixer’s bowl above the water, but not touching the water.
Whisk the sugar, egg whites, vanilla and salt in the stand mixer bowl. Set the bowl above the boiling water and continue to whisk constantly until the sugar has dissolved and the egg whites are slightly frothy, 3 to 4 minutes. Transfer to the stand mixer fitted with the whisk attachment and beat at medium-high speed until the meringue is cool and can hold very stiff peaks, about 6 minutes.
Transfer the meringue to a pastry bag fitted with a large round piping tip (see Cook's Note). Pipe 2- to-3-inch mounds all around the cake rim and in the middle, leaving a pointy tip at the top of each for the ghost head, about 14 total. Using tweezers or your fingers, add 2 mini chocolate chips, pointed-side out, to each face for the eyes; and 1 chip, pointed-side in, for the mouth.
Meringue is best used right after you make it. If you fill your pastry bag with only some of the meringue, then keep the remaining meringue in the stand mixer mixing on medium speed until you are ready to use it.