Preheat your oven to 180 C. Grease a round pan ( I used a 7 inch loose bottomed pan) with butter and dust with flour/cocoa well.
Melt the butter in a saucepan.
Get off the stove and stir in sugar and cocoa powder.
Stir in the eggs and the vanilla.
Mix in the flour.
Pour batter into prepared pan and bake for 25-30 minutes .
Cool in pan . Run a knife or spatula along the inside edge of the pan to loosen cake. Invert onto cooling rack .
Dust with icing sugar.
Notes
You can make the batter ahead of time and keep it the refrigerator for upto 2 days. Bake when you need it. You can also make it earlier and microwave whole cake or slices before you eat. But only 10 seconds or you ll have a plate of lava!
A 6 inch pan will take 35-40 minutes and 8 inch pan will take 22-24 mins. I suggest a springform or loose bottomed pan.
Do not over bake. No point in the toothpick test. When you press on the top of the cake , it'll slightly spring up.