Step 1: Preheat the oven to 190°C/375°F/Gas 5. Brush a 30 x 20cm tin
Step 2: with oil and line the base with non-stick baking parchment.
Step 3: PurÃ©e half of the raspberries in a food processor, reserving
Step 4: the other half. Mix the purÃ©ed raspberries with the egg yolks,
Step 5: caster sugar, yogurt and oil. Sift in the flour and cocoa and
Step 6: gently fold into the raspberry mixture.
Step 7: In a medium bowl, whisk the egg whites until they form soft
Step 8: peaks. Add a spoonful to the raspberry and chocolate mixture
Step 9: and stir well to loosen. Then gently fold in the rest of the egg
Step 10: whites and remaining raspberries.
Step 11: Spoon into the tin and bake for 20 minutes or until just firm
Step 12: to the touch. Cool for 5 minutes, then turn out onto a rack,
Step 13: removing the baking paper. Cut into squares to serve. Store
Step 14: in an airtight container.