Method

1. Line a 8cm-deep, 8.5cm x 17.5cm (base measurement) loaf pan with 2 layers of plastic wrap, allowing it to overhang sides. Spoon half of the ice-cream into loaf pan and spread out evenly. Put loaf pan, flat, in the freezer for 2 hours or until ice-cream is firm.

2. Put strawberries, 50g of the raspberries, half of the yoghurt and 2 Tbsp of the sugar substitute in a small food processor. Process until smooth. Pour berry mixture over ice-cream. Return to freezer for 2 hours or until just set.

3. Spoon remaining ice-cream over berry mixture, then spread out evenly with the back of a spoon. Return to freezer for 2 hours or until firm.

4. Put remaining raspberries, remaining yoghurt and 2 Tbsp of the sugar substitute in food processor. Process until smooth. Pour berry mixture over ice-cream, then spread out with the back of a spoon. Return to freezer for 2 hours or until firm.

5. Halve sponge through centre to form 2 thin rectangles, trimming one half to fit loaf pan. (Keep remaining half for another use.) Whisk cocoa powder, remaining sugar substitute and boiling water in a shallow dish. Dip each side of sponge in cocoa mixture, allowing it to absorb as much liquid as possible.

6. Carefully place sponge in loaf pan. Cover sponge layer with a piece of freezer wrap or plastic wrap to seal. Wrap entire loaf pan with a double layer of plastic wrap to prevent sponge from drying out. Return to the freezer for 6 hours or until set.

7. Remove plastic wrap from loaf pan and piece covering sponge. Wipe a warm cloth over base of pan, then lift and remove cake from pan using the inner layers of plastic wrap. Turn over onto a platter and remove plastic wrap.