Preheat your oven to a delightful 150°F and prepare a 7-inch springform pan. Line the bottom with baking paper and grease the sides with a bit of butter.
In a bowl, sift together the flour and cacao, repeating the process to ensure any lumps have been discarded. Add in egg yolks, half the sugar, melted butter, milk, and vanilla to the mix.
In a separate bowl, beat the egg whites with a pinch of salt until soft peaks form. Add a few drops of lemon juice and start to gradually add sugar, one tablespoon at a time. Beat until stiff peaks form.
Gently incorporate the egg whites into the cacao batter and stir delicately to keep the air bubbles inside. Pour the batter into the springform and bake for an 85 minute period.
Remove the cake from the oven and cool it on a kitchen counter. Once cooled, transfer it onto a cake stand.
Prepare the topping; melt the chocolate and butter over steam. Pour the melted mixture over the cake and place it in the fridge until the chocolate is firm.
Slice and serve your freshly-made cake!