Wash 250 g blueberries, drain well. Roughly chop 125 g wafer rolls or cut into small rolls. Cream egg yolk, sugar and 1 sachet of vanilla sugar. Stir in buttermilk and lemon zest. Whip 500 g cream until stiff and also stir in.
Cover the mixture and put it in the freezer for 1 1/2-2 hours and let it freeze a little. Stir the mixture every 30 minutes. As soon as the mixture begins to freeze completely, fold in blueberries and rolls. Spread the mixture into a silicone ring cake mould (22 cm Ø; approx. 2.75 litres capacity). Cover and freeze for about 12 hours, preferably overnight. Sort 50 g blueberries, wash if necessary and drain well. Crumble the remaining rolls. Whip 125 g cream and 1 packet of vanilla sugar until stiff and place in a piping bag with a star-shaped spout. Turn out the ice cream cake from the tin.
Sort 50 g blueberries, wash if necessary and drain well. Crumble the remaining rolls. Whip 125 g cream and 1 packet of vanilla sugar until stiff and place in a piping bag with a star-shaped spout. Turn out the ice cream cake from the tin. Decorate with cream tuffs, blueberries and roll crumbs. Serve immediately