1. Preheat oven to 450°.
2. Lightly spoon flours and cornmeal into dry measuring cups; level with a knife. Combine flours, cornmeal, sugar, baking soda, baking powder, and salt in a large bowl; stir with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add buttermilk; stir just until moist.
3. Turn dough out onto a lightly floured surface. Roll dough to a 3/4-inch thickness; cut with a 2 1/2-inch biscuit cutter into 18 biscuits. Gather remaining dough. Roll to a 3/4-inch thickness, and cut with a 2 1/2-inch biscuit cutter into 6 biscuits. Place 12 biscuits on a baking sheet coated with cooking spray. Brush tops of biscuits with half of egg white. Combine poppy seeds and sesame seeds in a small bowl, and sprinkle half of seed mixture evenly over biscuit tops. Bake at 450° for 10 minutes or until biscuits are golden; place on a wire rack. Repeat with remaining 12 biscuits, egg white, and seed mixture.