Preheat the oven to 170 degrees Celsius. Grease a 7- or 8-inch baking tin and line the bottom with parchment paper. Set aside.
Sift the flour and cocoa powder together thrice. Set aside.
Place the eggs in a medium pot or a large metal mixing bowl. Add in the sugar and whisk immediately until combined. You must whisk the sugar immediately into the eggs otherwise lumps will form and it will be very hard to get rid of them later on.
Hold the pot / mixing bowl over medium-low heat and whisk the egg-sugar mixture constantly, to bring up the temperature until its like baby's milk - warm but not hot. The mixture would be foamy. You can set it directly over the heat, just make sure that you remove it from time to time so you don't end up with cooked eggs.
Transfer the mixture in the pot into a mixing bowl and whisk it with a hand-held electric mixer on high speed until the bubbles start to disappear. The mixture will triple in volume. Turn the mixer down to low and whisk it for at least 1 minute, to stabilize the air bubbles in the cake batter. The cake batter will leave a trail when you lift up the beaters and will sink and merge back slowly into the batter.
Sift in half of the flour mixture and gently fold it in until almost all are combined. Sift in the remaining half of the flour mixture and fold until no flour pockets are visible. Do not over-mix.
Add one-third of the cake batter into the hot, melted butter. Use a smaller spatula and fold the butter into the cake batter thoroughly. Pour the butter-mixed batter back into the rest of the cake batter and fold in gently.
Transfer the cake batter into the baking tin. Tilt the tin so the batter spreads out evenly and drop it two to three times on the counter-top to burst the uneven air bubbles.
Bake in the middle rack for 15 to 20 minutes until the cake springs back when touched gently and a skewer inserted into the centre of the cake comes out clean.
Let the cake cool completely in the tin before decorating / frosting it.