First of all preheat the oven to gas mark 3 or 170C. If you have an electric fan oven, you might need to adjust the temperature to 150-160C and take the cake out a little earlier than 45 minutes (I normally check mine after 30 min)

Melt the butter in the microwave or on the hob, add the sugar and mix well.

Add the flour and cocoa powder and mix until you have a fairly stiff mixture.

Beat one egg separately in a bowl and gently mix into the flour mixture.

Pour the mixture into a greased and lined 30cmx18 cm baking tray and spread evenly.

Spray the top with a bit of water and bake for about 45 minutes.

Once done, sprinkle with more sugar and allow to cool for a bit before serving with a custard or cream.