Preheat oven to 400. Place a baking sheet into the oven. Grease an 8 inch cake pan.
Place mashed potatoes in large mixing bowl. In a separate bowl, sift the flour and baking powder. Put the butter in a small bowl and mix in dried herbs. Add to mashed potatoes and work the two mixtures together gradually adding just enough milk to bind, you may not need much at all, depends on how dry the potatoes are. Turn the dough out onto a lightly floured board and knead very lightly until pliable. Roll into a ball, flatten and roll out into a circle about 8 inches in diameter. Transfer to prepared cake pan. Cut into eight wedges and place in the oven. Brush with beaten egg to glaze, if desired. Bake for 25 minutes until golden brown. Test.