1Preheat the oven to Gas 4, 180 C. Grease and line the swiss roll tin with baking parchment.
2Cream the butter and sugar together until light and fluffy, add the eggs on at a time and beat well after each addition. Add the chocolate and beat again. Sieve the flour and baking powder and fold into the mixture.
3Place into the tin and spread out evenly, pushing well into the corners. Bake for 25 minutes until well risen and springy to the touch. Cool on a wire rack.
4Cut the sponge into 3 equal rectangles. Whip the cream with the vanilla and sugar until holding shape and then use with the strawberries to sandwich the sponges together. Dust the top layer of sponge with cocoa powder.