Heat the oven to 356°F (180 °C).
Use a mixer to beat 4/5 cup (200 g) of softened butter with the sugar, add the eggs 1 by 1, the sifted flour, starch, salt, baking powder.
Mix 4 tbsp of cocoa with the milk and add this to the dough. Distribute it into a buttered form about 10″ (26 cm) in diameter.
Bake in the middle of the heated oven for about 35-40 min.
Stir the liquid chocolate with the rest of the butter. Take the cake layer out of the oven, leave it to cool outside the form.
Once cooled, cut it into 2 and distribute the chocolate (set aside 2 tbsp of it) over one of the layers. Place the 2nd layer on top, distribute the rest of the liquid chocolate in a thin layer and sprinkle with cocoa.
Leave the cake in the fridge for at least 1 hour.