Preheat oven to 350°F (180°C).
Put foil paper in a loaf pan. My loaf pan is 8*4*3 inch (L*W*H). It's slightly bigger than a whole block of chocolate.
Place 30g butter in a microwave-safe bowl, microwave for 20 seconds, take out to stir until fully melted. Set aside. (Or melt the butter over hot water bath if you don't have a microwave.)
Add 3 eggs and 3 tbsps of sugar to a mixing bowl, beat the eggs with a hand mixture. Use medium speed to beat for about 3 minutes, until the mixture turns fluffy, creamy, and pale. (*See Note 1 for the egg beating tip.)
Add ⅓ cup of cake flour and 4 tbsp of cocoa powder to egg mixture. Blend until fully combined.
Add melted butter to the mixture. Blend until fully combined.
Pour the mixture to the loaf pan.
Bake at 350°F (180°C) for 20 minutes.
Take out the cake. Set oven temperature at 220°F (105°C).
Remove foil paper from the cake, put the cake on an oven-safe plate, place a whole block of chocolate on top of the cake.
Bake the cake at 220°F (105°C) for 2 minutes.
Take out the cake, serve hot.