Preheat the oven to 350°F (175°C). Coat three 8-inch round cake pans with baking spray or butter, then line the base of each with parchment rounds. Spray or butter the parchment. Coat the interiors of the pans with cocoa powder (use about 1 tablespoon and transfer the residual cocoa powder from pan to pan).

In a medium bowl, whisk the flour, cocoa powder, baking soda, baking powder, and salt.

In a small heatproof bowl pour boiling water over chocolate. Cool for 15 minutes, stirring until the chocolate melts.

Using a stand mixer fitted with the whisk attachment, whisk the eggs on high speed until light and foamy (about 2 minutes). On low speed, add the oil, then increase the speed to thoroughly combine.

On low speed, add the chocolate mixture and beat briefly. Add the buttermilk and vanilla and beat briefly to combine. Add the sugar and beat until the batter is smooth, about 2 minutes.

Remove the bowl from the stand mixer and gently fold in the dry ingredients with a soft spatula until combined, pressing against the sides of the bowl to break down large lumps. The batter will be slightly lumpy.

Divide the batter evenly among the prepared pans. Bake for 55-65 minutes or until the tops spring back when lightly pressed and a toothpick inserted in the center comes out clean.

Cool the cakes in the pans for 20 minutes, then run a knife around the edges to loosen. Remove cakes from pans and cool on racks completely.

In the bowl of a stand mixer fitted with the whisk attachment, combine the egg whites and vanilla. Beat just until frothy, then stop.

In a small heavy saucepan, combine the sugar, cream of tartar, and water and bring to a boil. Continue boiling until the sugar reaches 175°F on a candy thermometer. Remove from the heat immediately.

Turn the mixer back on medium high speed, beating the egg whites. Slowly pour the sugar syrup into the whites (aim for between the whisk and the side of the bowl). Beat until the whites are very fluffy, glossy, and cooled almost completely. Add drops of pink food coloring, if using, until you get the depth of color you like.

Arrange one of the cake layers on a cake platter. Spread a generous layer of frosting on top. Arrange a second layer evenly on top of the frosting and frost the top of it with the same amount and thickness as the first. Arrange the third layer on top of that and frost the top and sides of the cake. Serve within a few hours.