Preheat the oven to 374°F (190° C). Lay out a square oven dish with baking paper and then brush it with a little melted butter. In a pan on the stove, melt the chocolate, butter and sugar over low heat, stirring the entire time until the mixture becomes smooth.
Allow the mixture to cool and add eggs and vanilla. Stir well and slowly add sifted flour and cocoa. Stir again and pour the mixture into the prepared baking dish.
Put it to bake for 20 minutes, remove and allow to cool.