Preheat the oven to 180°C/350°F/Gas mark 4.
Grease the base and sides of a 25 x 38cm (10 x 15 inch) Swiss roll tin or roasting tin with the same base measurement and line the base with a piece of nonstick baking paper. Add the margarine and sugar to the bowl of an electric mixer or food processor, sift in the flour, cocoa and baking powder then add the eggs and beat until smooth, or mix in a bowl with a wooden spoon.
Spoon the cake mixture into the lined tin and spread into an even layer. Bake for 15-18 minutes until the cake springs back when lightly pressed in the centre of the tin with fingertips. Leave to cool for 15 minutes then loosen the edges and turn out on to a wire rack.
When cold, peel off the lining paper and fold the paper into three triangles, with the two outer triangles going one way and the centre one pointing the other. Draw a large heart in one of the triangles that is about 14 x 20 cm (5.5 x 8 inches) then cut out, and use this as a template to cut two more hearts from the other triangles. Lay the templates on the cake and cut around them with a small sharp knife. Check the shapes are the same by stacking the cakes one on top of the other and trimming a little off the sides if needed, then separate the cakes once more.
Melt the chocolate in a bowl set over a saucepan of very gently simmering water for about 5 minutes. Stir until smooth then take off the pan. Slowly whisk in the cream until thick and glossy then chill in the fridge for about 15 minutes until thick enough to spread.
Place raspberries around the edge of one of the cakes, then crumble the rest and arrange with the sliced strawberries over the other two layers. Carefully stack the cakes together on a pedestal plate, if you have one, with the raspberry edged cake on top then dust the raspberries with a little sifted icing sugar. Chill in the fridge until ready to serve.