Set the oven to 190C/375F/Gas Mark 5. Line a tray of bun tins with paper cases or with paper muffin cases.
Sift together the flour and baking powder. Add the sugar, margarine and eggs and beat together until pale and glossy.
Divide the mixture in half. Leave one half white and add the cocoa powder to the other half.
Put a spoonful of plain mixture in one half of each paper case and a spoonful of chocolate in the other. Swirl together with a skewer or cocktail stick.
Bake for 15 - 20 minutes until well risen and firm to touch. Place a chocolate button on top of each cake as soon as they come out of the oven.
Leave small strawberries whole or cut large ones in half. Once the chocolate buttons have softened with the heat of the cakes put a strawberry on top. Cool on a wire rack until the chocolate has set.