Place the flour, salt and sugar in a bowl. Crumble the yeast, add to the flour and mix in. Add the milk, butter and egg, knead to form a soft, smooth dough using the dough hook on a food processor. Cover the dough and leave to rise at room temperature for approx. 3 hrs. until doubled in size.
Pit the cherries, roughly chop. Place the biscuits in a plastic bag, crush with a rolling pin, mix with the cherries.
Roll out the dough into a 30 x 40 cm rectangle. Spread the filling on top, roll both sides of the dough in towards the middle. Cut into approx. 2,5 cm pieces, place on two baking trays lined with baking paper. Leave to rise for a further 20 mins.
Approx. 20 mins. in an oven preheated to 180 °C (convection). Remove from the oven, slide onto a rack.
Combine the lemon juice and icing sugar. Drizzle over the still-warm butterfly rolls, leave to cool on the rack.