Cut the bananas into 2 cm (3/4 inch) slices, and place in a resealable plastic bag. Expel the excess air, then tightly seal. Freeze for about 6 hours, until firm.
Meanwhile, line a sieve with muslin (cheesecloth) and stand over a bowl, with the bottom of the sieve well clear of the base of the bowl. Spoon the yogurt into the muslin, gather up the ends and twist to enclose. Place a saucer on the muslin and weigh down with two 400 g (14 oz) cans. Place in the fridge to drain for about 4 hours.
Combine the frozen banana slices, frozen raspberries, drained yogurt and vanilla in a food processor. Process until smooth and evenly combined, occasionally stopping and scraping down the side with a rubber spatula.
Transfer the mixture to an airtight container and freezer for about 6 hours, until firm. Remove from the freezer 10 minutes before serving, so it can soften. Alternatively, for individual serves, freeze the mixture in 8 x 100 ml ice-block popsicle moulds.
Each serving provides: 998 kJ 238 kcal 14 g protein 5 g fat (4g saturated fat) 34 g carbohydrate 25 g sugars 3 g fibre 236 mg sodium