Melt the butter and crush the biscuits. Mix the butter and biscuit crumbs together. Cover the bottom of a 16-cm springform cake tin with parchment and fill with the crumbs, using the back of a spoon to press them down firmly against the bottom of the tin. Place the cake tin in the freezer briefly to harden the biscuit layer.
Meanwhile, make the filling. Soak the gelatin in cold water for 5 minutes. Heat the lemon juice in a saucepan, squeeze out the gelatine and dissolve it in the juice. In a large bowl, stir the lemon juice and gelatine through the Greek style yoghurt. Add 2 tablespoons of sugar if desired. Retrieve the cake tin from the freezer. Pour the filling over the biscuit layer, smoothing the top with a spatula. Put in the refrigerator to set for two hours. Garnish with fresh raspberries.