In a mixing bowl, without adding water, thoroughly blend ww flour, durum flour and sea salt.
Heat 1 1/4 cups water to about 90F or lukewarm. Pour water into 500 ml or larger jar. Add 1 heaping tspn sugar and dissolve. Add 1 tspn yeast and stir in. Let sit 20 minutes in a warm place til the yeast is vigorous and foaming well over the water.
Add mashed potatoes to flour blend, and yeast-water. Stir with wooden or plastic paddle til it is the consistency of wet paste. Slowly sprinkle in more ww flour and stir til it is more like a damp putty that you can then work by hand.
Dust hands in ww flour and knead for five minutes in bowl, thoroughly blending yeast-water and mashed potatoes with all the other ingredients. If some dough does not stick to your hands, it is too dry. If so, carefully add just a little more warm water. It should be the consistency of damp putty.
After 5 minutes kneading, cover the bowl so it is sealed and place in refrigerator. Let chill for 12 hours. This encourages a glutinous texture.
The next day, remove from fridge and remove cover. Cover with wet towel and place in warm area to rise and ferment for 6 to 8 hours.
Preheat oven to 400F.
Dust a work area with ww flour. Gently remove from bowl onto dusted surface. Flip so top and bottom now have dusting of ww flour. Gently shape dough into a rounded cake. The dough should be air filled, rubbery and glutinous now. Pinch outside and pull out then wrap over, drawing the stretched dough toward the center. Do this all around the dough ball, reforming the ball. This encourages air in the dough and develops the gluten.
Lightly oil a baking sheet and gently place ball of dough on it. It should 8 to 10 inches in diameter at this point. Gently round the sides. Score the top with a sharp knife in a cross pattern.
Place two ice cubes in a small, metal bowl and place in a corner of the oven. This will create humidity that helps form a chewy crust.
Dust the top of the dough with ww flour.
Spray the dough ball with cold water.
Place in oven and let bake at 400F for 40 minutes.
Remove and let cool 20 minutes before serving.