In a bowl, combine the flour with all of the other ingredients up to and including the yeast, add the butter and milk, knead to a soft, smooth dough using the dough hook on a food processor for approx. 10 mins. on a low setting. Cover and leave to rise at room temperature for approx. 2 hr. until doubled in size.
Simmer the rhubarb, berries and sugar in a pan for approx. 10 mins., allow to cool. Stir in the coconut flakes and hazelnuts.
On a lightly floured surface, roll out the dough into a rectangle approx. 30 x 40 cm, place on a sheet of baking paper. Along the shorter two edges, cut strips approx. 10 cm long and 1 cm thick. Spread butter, nuts and chocolate onto the surface which has not been sliced. Place the rhubarb and berry filling on top. Fold the cut strips over one another from the right and left sides. Brush with a little egg, scatter over the sugar.
Approx. 35 mins. in the centre of an oven preheated to 180°C. Remove from the oven, transfer to a cooling rack and allow to cool. Dust with icing sugar.