Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Lightly grease a 7cm (2 ¾ inch) deep, 450g (1lb) loaf tin. To make the heart centre; Put the margarine in a bowl with the caster sugar and eggs. Sieve the flour and Baking Powder on top. Carefully mix all the ingredients together then whisk for a few seconds until well blended.
Mix in sufficient Red Gel Food Colour to make a vibrant shade (you will need approx. 90 drops), and spoon into the loaf tin. Smooth the top and bake for about 1 hour until risen and firm to the touch – a skewer inserted into the centre should come out clean. Cool for 5 minutes, before turning on to a wire rack to cool completely.
Put the cold loaf cake on a board and slice off both crusty ends. Cut the loaf into 7 slices. Using a 7.5cm (3inch) wide heart shaped cutter, cut out 1 heart from each slice – keep all the trimmings for cake pops or truffles. Place on a tray lined with baking parchment and freeze the hearts for 30 minutes.
For the chocolate cake, wash out the loaf tin and re-grease. Put the margarine in a bowl with the caster sugar, eggs and Vanilla. Sieve the flour, cocoa and Baking Powder on top. Carefully mix all the ingredients together then whisk for a few seconds until well blended.
Thickly spread some of the cake mixture over the bottom and sides of the tin, making sure it is completely covered. Arrange the sponge hearts, upright, down the length of the tin, closely together so that they support each other.
Carefully spoon the remaining cake mixture on top of the hearts, gently tapping the tin on the work surface to help the mixture settle round the hearts. Make sure the hearts are well covered in cake mixture to help create the surprise.
Put the tin on a baking tray and bake in the oven for about 50 minutes until risen and firm to the touch, and a skewer inserted into the centre comes out clean. Cool for 10 minutes before removing from the tin, and leave to cool completely on a wire rack.
To decorate, break the Extra Dark and Milk Chocolate into a small, heavy bottomed saucepan. Add the butter and 15ml (1 tbsp) water. Heat very gently, stirring occasionally, until melted together and smooth. Remove from the heat and set aside for about 20 minutes to cool to a thick, coating consistency.
Put the loaf cake on the wire rack over a tray. When the chocolate mixture starts to thicken, spoon over the top of the loaf. Gently tap the wire rack on the tray to spread and smooth the icing on top, letting it drip down the sides of the cake. Leave to stand in a cool place for several minutes to allow the icing to set before transferring to a serving plate. Your cake is ready to slice and surprise!