Preheat oven to 350º F.
Sift flour, baking powder and salt together into a large bowl.
Add butter and by hand work it into the flour mixture until it becomes a coarse mealy texture.
Cook sweetpotato in boiling water until softened. Cool, remove skin and purée with milk to make a smooth mixture.
Beat 1 cup of the sweetpotato purée into flour and then turn out onto a lightly floured surface.
Knead well about 80 turns. Divide dough into 4 pieces and roll each very thin. Cut into rounds or squares; sprinkle with salt, sesame seeds and a little cayenne, if desired.
Bake in oven on an ungreased cookie sheet for 10 minutes until bottoms are slightly browned. Turn over and bake 3-4 minutes more. Cool on rack before storing in a airtight container.