Prepare mashed potatoes by boiling peeled/quartered potatoes until tender (approx. 20 minutes) and mashing with 2 tablespoons milk.
Prepare dough sponge and proof yeast by mixing yeast, 1/2 cup flour, and 1/2 cup water with 1/2 teaspoons honey dissolved in the water in a bowl and letting stand for 20 minutes until mixture is foamy. (Throw out and start over with fresher yeast if not foamy.).
Mix 1/2 of remaining flour with herb mix, salt, remaining water with remaining honey dissolved in it, olive oil, dough sponge, and mashed potatoes. Mix in remaining flour (dough should only be slightly sticky). Knead 15 minutes on a floured surface (10 minutes in a machine) until smooth.
Place dough in lightly oiled bowl and cover with plastic wrap. Let rise until doubled (about 1 to 1 1/2 hours) in a warm place. *Punch dough down and divide in half. Put dough halves in greased 9-inch cake pans and spread over bottom. Cover with towel and let rise until doubled.
Dimple the dough surface with fingers. Drizzle with extra-virgin olive oil** and sprinkle with coarse salt. Bake in preheated 400°F oven with pans directly on the pizza stone for 25-30 minutes or until golden brown. Remove from pans and place on a wire rack.
Cut in wedges and serve warm or at room temperature with dipping plates of extra-virgin olive oil (3 Tbsp) and balsamic vinegar (1 tsp).
Focaccia Tips: Let dough rise until it falls (approx. double the time) and knead in extra flour. This technique improves the texture and flavor of the focaccia. Substitute other toppings such as a herbed garlic oil, sun-dried tomatoes, Greek olives and walnuts, etc. Add fresh herbs only after baking.