Mix the flour, salt, sugar and yeast in a bowl. Add the butter, milk and egg, knead to form a soft, smooth dough. Cover and leave to rise at room temperature for approx. 2 hrs. until doubled in size.
Divide the dough in half. On a lightly floured surface, roll out the dough into two rectangles (each approx. 65 x 20 cm). Cut both rectangles in half lengthwise, spread the butter over the pieces of dough, sprinkle each with approx. ¼ of the chocolate. Starting from the long edge, roll up the pieces of dough. Cut the rolls in half crosswise, then lengthwise. Twist 2 strands of each in a spiral fashion and shape into buttons. Place all eight buttons on a baking tray lined with baking paper, cover and leave to rise for a further 30 mins.
Approx. 20 mins. in the lower half of an oven preheated to 180°C. Remove from the oven, slide onto a cooling rack and allow to cool. Dust with icing sugar.