This is cooked in an ungreased tin and must not be turned out until it is cold or it will break up. Ice top then cut into slices.
Oven temperature 400F (I cook it at 200c)

1.5 cups plain flour
1 cup sugar
1 rounded tsp baking powder
½ tsp Bicarbonate of Soda
2 tablespoons Cocoa

Sift all these together and put into a tin 8x10 inches (20x25cm) or 8x12 inches (20x30cm).
Melt 5oz (150g) of margarine. Make three dents in the dry ingredients and put 1 tsp of vanilla in one dent, 1 tablespoon of white vinegar in the next and the melted margarine in the last. This will run all over. Then pour over 1 cup of tepid water and mix very well. Bake for ½ hour.