Combine 1 cup whole wheat flour, water, and sourdough starter in a large bowl. Stir thoroughly and cover with a clean towel. Let sit until spongy, 7 hours to 1 day.

Stir remaining 1/2 cup whole wheat flour, amaranth flour, honey, baking soda, and salt into the starter mixture. Mix in all-purpose flour, 1/2 cup at a time, until the dough is no longer sticky. Knead gently until dough is smooth and elastic.

Roll out dough on a floured work surface to 1/2-inch thickness. Cut into rounds with a 2 1/2-inch biscuit cutter. Sprinkle some cornmeal over a baking sheet. Transfer rounds to the baking sheet; sprinkle tops with remaining cornmeal. Cover with a clean towel and let rise until puffy, about 45 minutes.

Heat a large skillet over medium heat. Place rounds into hot skillet and lower heat to medium-low. Cook until golden brown and edges look dry, about 6 minutes per side. Transfer to a wire rack to cool completely.