For the bread dough, dissolve yeast in warm water.
Heat milk, 85g butter, mashed potato flakes, sugar and salt to 55-57° C/110-115° F. Cool for at least 15 minutes. Stir in the yeast mixture and the beaten egg. Gradually add the flour until a stiff dough ball is formed, you may not need all the flour. On a floured surface, knead the dough until smooth and elastic, approximately 6-8 minutes. Place the dough in a greased bowl, turning once to grease the top. Cover and let the dough rise in a warm place until doubled, approximately 1 hour.
For the filling, in a double boiler, or a heatproof bowl over a saucepan of simmering water, combine the filling ingredients. Cook over low heat until thickened, stirring constantly. Cool.
Punch dough down. Turn onto lightly floured surface. Divide the dough in half. Roll one portion into a 30x40cm/12x16” rectangle. Spread half of the filling over the dough within 1.5cm/1/2” of the edges. Roll up jelly-roll style, starting with the long edge. Seal edges by pinching the seam. Place seam side down on a greased baking sheet; pinch the ends together to form a ring. Using a sharp knife, cut 2.5cm/1” slices almost through the ring. Brush remaining butter on the top of the ring.
Repeat steps above with the remaining dough and filling. Cover the rings and let them rise in a warm place until doubled, about 45 minutes.
Bake at 180°C/350°F until lightly browned, 20-25 minutes. Remove from pan to wire racks to cool.
These can be frozen. Thaw and warm in the oven before serving.