Preheat oven to 325 degrees Fahrenheit.

Have tube pan ready. Do NOT grease.

In a small bowl, whisk together the cocoa powder and boiling water until smooth. Set aside to cool to room temperature.

In a mixing bowl, whisk together the flour, sugar, and baking powder.

Pour the egg yolks, melted butter, and vanilla extract into the dry ingredients and mix just until combined. It will be stiff and dry, that's okay.

Add the cooled cocoa mixture and stir until combined.

In a large bowl (or the bowl of stand mixer), combine the egg whites and cream of tartar. Beat until stiff peaks form.

Pour the chocolate mixture over the egg whites and fold gently until completely combined.

Pour the cake batter into the tube pan.

Bake in the preheated oven for 55 minutes.

Turn the oven temperature up to 350 degrees Fahrenheit, bake an additional ten minutes.

Test the cake to make sure it's done by inserting a tester or thin knife - it should come out clean.

Invert the pan to cool, either placing it on its feet or inverting over a bottle if there are no feet. Let cool at least one hour this way.

Once cool, gently run a butter knife around the edges of the cake and pan to loosen. Push up the center to remove the outer pan.

Run the butter knife around the tube center and underneat the base of the cake.

It should come loose from the pan and turn the cake over onto a cake plate to finish cooling.

In a small saucepan, combine the butter and cocoa. Whisk together until the butter is completely melted and a thin chocolate sauce is formed.

Remove from heat.

Add vanilla extract and powdered sugar. Stir and add 1 Tablespoon of Milk. Whisk until smooth adding more milk a little as time to keep a smooth, thin glaze.

Pour the glaze over the cake, using an offset spatula or butter knife to force edge over the edges of the cake.