Preheat the oven to 355 degrees F (180°C). Butter and dust an 8-inch (20 cm) round cake pan with flour, and remove flour excess. Or butter and line the pan's bottom with a parchment paper circle and the sides with parchment strips.

Melt the butter in the microwave at 30-second increments and let it cool to 122-140°(50-60°C). Combine cocoa powder with flour, sift two-three times, and set aside.

Prepare a bain-marie or water bath by pouring some water into a small saucepan and bringing it over medium heat. You can use a store-bought double boiler.

Place eggs and sugar in a larger heatproof bowl or the bowl of a stand mixer, and whisk eggs with sugar with a hand whisk a few times. Place the bowl over the saucepan with the hot water, ensuring the bottom doesn't touch the water. Whisk the egg mixture until frothy and whitish. Use a cooking thermometer to check the temperature reaches 104°F (40°C).

If you don't have a thermometer, test the heat by comparing it to the warmth of the body temperature of 98.6°F (37°C). It should feel warm but not hot to the touch. Be careful with steam coming from the hot water and touching the batter.

Remove the bowl from the heat, and beat the preparation for 8 minutes. Use a stand mixer with the whisk attachment or an electric mixer on high or medium-high speed. Decrease the mixer speed and beat for another 2 minutes. Long whisking will result in a tripled volume and reach the ribbon stage (the batter forms thick trails when lifted with the whisk).

Add 1/3 of the sifted flour mixture to the wet ingredients and delicately mix with a large rubber spatula from the bottom to the top. Mix another third of the dry ingredients, and finish with the rest of the flour mixture, gently mixing with a spatula. 

Add two spoonfuls of the batter to the melted butter and mix with a spatula. Gently pour the blended butter mixture over a spatula into the remaining batter and carefully mix. 

Pour the batter into the prepared pan and level the top using a spatula or spoon. Bake for 30-35 minutes. Test the doneness by inserting a toothpick in the center; it should come out clean. Watch for the cake that slightly shrinks from the edge of the pan.

Remove the cake from the oven and let it rest in the pan for 5-10 minutes. Then flip it onto a wire rack, and cool it for 5-10 minutes. Finally, turn it over again and let it cool completely.