Separate the egg whites about an hour ahead and let them come to room temperature.

Preheat the oven to 375°F conventional or 350°F convection with the rack in the middle. Confirm the tube pan will fit with space above it.

Sift together the cake flour, powdered sugar, and cocoa. Set aside with the sifter — you will sift again directly over the batter.

Beat the egg whites and cream of tartar on medium speed until foamy.

Increase to high speed and add the granulated sugar and salt about 2 tablespoons at a time, no more than 15 seconds between additions. Beat until stiff, glossy peaks form.

Add the vanilla extract and coffee extract. Mix briefly on low, then increase to high speed.

Sift the flour-cocoa mixture over the meringue in approximately ¼ cup increments, folding gently after each addition until almost incorporated. A few streaks are fine. If the batter begins to look liquid, stop folding immediately and transfer to the pan.

Push the batter into an ungreased 10×4 in tube pan. Run a knife or small offset spatula through in a swirling motion to release any large air pockets.

Bake 30 to 40 minutes until the top springs back and a tester inserted into a crack comes out clean.

Immediately invert the pan onto its built-in legs or over a bottle and cool completely before releasing.

To release, run a small offset spatula all the way to the bottom around the outer edge, then around the inner tube. Flip carefully onto your hand while holding the center tube — it should release cleanly without forcing.

Combine the cream, milk, half the granulated sugar, salt, and vanilla bean seeds and pod in a heavy-bottomed saucepan. Bring to a boil, then remove from heat.

While the milk mixture heats, whisk together the eggs and remaining sugar until lightened to pale yellow.

Whisk ¼ of the hot milk mixture into the eggs, then another ¼, then pour the egg mixture back into the pot with the remaining milk.

Cook over medium-high heat, stirring constantly, until the custard reaches 178°F or is nappant — it holds a clean line when you drag a finger across the back of a coated spoon.

Strain immediately into a bowl, press plastic wrap directly onto the surface, and cool over an ice bath.

To serve, spoon a smooth pool of cold crème anglaise onto the plate and set a slice of cake alongside.