The night before, wrap the potato in plastic wrap then microwave on high for two minutes followed by ten minutes at low power. Remove the skin from the potato and put the flesh in a medium mixing bowl. Add the butter in pieces and let melt. Use a mixer to mash the potato thoroughly. Dissolve the yeast in a half cup of the water. When the potato is cool enough not to burn, add the water and yeast and mix on high speed. Stir in the salt, sugar, baking powder, and flours. The batter should be thick enough to hold a ribbon when a spoon is dragged across it, but not so dry that it breaks up when stirred. Add water as needed to get the right consistency. Cover the batter and put it in a cool place overnight. In the morning, preheat the waffle iron. Separate the egg. Fold the yolk into the risen batter. Beat the white until stiff peaks are formed. Fold this gently into the batter. Spoon the batter into the waffle iron, pushing it into the corners. Cook according to the iron's directions. Makes about eight Belgian-style waffle squares.