In a small saucepan, heat the milk until bubbles appear around the edge. Remove from the heat and add the yeast. Let stand until foamy, about 5 minutes.
In a large bowl, combine 2/3 cup of the flour with the cornstarch, soy sauce, baking powder, cayenne and 1/2 teaspoon of salt. Stir in the yeast mixture. Cover and let stand in a warm place for 1 1/2 to 2 hours.
In a large skillet, heat 2 inches of oil to 350°. Season the cod with salt and black pepper. Dredge 1 fillet at a time in flour, tapping off the excess. Coat the fish in the batter and fry in batches until golden, about 4 minutes per side. Drain on paper towels and serve at once. Pass the Minted Pea Puree separately.