Preheat the oven to 160 °C (320 °F). Line a small 20/20 cm ( 8”x8”) baking tray with butter and flour (or parchment paper). Position the oven rack on level 3 from the top.
Combine the flour with the cocoa powder and a pinch of salt.
Melt the butter over a low heat and set aside. Whisk the egg yolks and sugar well until they double in volume and lighten in color. Add the hot water and mix for a few seconds. Add the melted butter and mix slowly.
Fold in the dry ingredients. Gradually pour in the milk and stir continuously to obtain a smooth mixture. Finally, add the amaretto essence and the scraped out inside of the vanilla pod.
Whip the 4 egg whites together with a pinch of salt until you get a firm foam. Incorporate 1/3 of the egg whites into the sponge mixture and stir well. Incorporate the rest of the foam gently, with upward movements. Don’t worry about making it completely smooth, then pour into the baking tray and place in the oven.
Bake for about 50 minutes, or until nicely browned on top. Let it cool in the tray.
Dust the magic cake with cocoa or sugar before serving.