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Heat oven to 180C/160C fan/gas 4. Grease and line the base of a deep 22cm round cake tin with baking parchment.
To make the raspberry sponge, put the yogurt, oil and raspberries in a food processor or blender, and whizz until well blended. Sieve, to remove the seeds, into a measuring jug, until you reach 300ml. Discard the rest.
From now, make the 2 cakes simultaneously. In 2 mixing bowls, separately mix the dry ingredients, plus a pinch of salt, for the raspberry and chocolate sponges. Crack in the eggs, then add the yogurt, oil and vanilla extract to the chocolate one, and the measured raspberry mixture to the other. Beat each with an electric whisk until smooth, cleaning whisks between bowls. Quickly stir some pink food colouring into the raspberry batter, to give a strong colour – a good cherry pink works well.
Sit the cake tin in front of you and a bowl of cake batter to each side – you need to work quickly and smoothly for the next bit, so make sure your oven is hot and racks are in the right place. Using 2 x ½ cup measurers, or 2 similar ladles or serving spoons, spoon 1 measure (about 150ml) of the chocolate batter into the very centre of the prepared cake tin. Don’t wait for it to settle or spread, quickly top with the same measure of raspberry mixture, right in the middle (see Secrets of success, below). Repeat alternating batters, without pausing, until all the mixes are finished, then put straight in the oven.
Bake for 1 hr until a skewer poked into the middle comes out clean – keep an eye on it though, you may want to loosely cover with parchment after 45 mins if the cake is looking dark (the top will be cracked). Cool in the tin.
To decorate, put the cream and raspberry jam in a pan, and bring nearly to the boil, stirring to melt the jam. Take off the heat, tip in the chocolate and leave until melted. Stir together until smooth. When thick enough to spread, remove the cake from the tin and swirl the chocolate icing all over the top.
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