In a large bowl, combine the corn flour and sugar. Gradually add the water, incorporating it into the dry ingredients with your fingers. Work in 3 tablespoons of the melted butter, kneading the mixture to form a soft dough. Knead in the cheeses, 1 cup at a time. If the dough seems too dry, add 1 to 1½ tablespoons more water. Taste for seasoning and, if necessary, add the salt to taste, 1/4 teaspoon at a time. Keep kneading the dough until it’s supple, smooth, and lump-free. It will feel like a cross between lukewarm mashed potatoes and Play-Doh.