Blueberry muffins - Testing

https://www.kingarthurflour.com/recipes/blueberry-sourdough-muffins-recipe

1 - I used some of the leftover from trying to start a sourdough starter and unlike the pancakes, this was not a success. The cornmeal, also, may have been too coarse. Anyway, the muffins are heavy and gritty. Is there anything else to do with this? Maybe buckwheat instead of cornmeal? Maybe let the dough stand as I do for the pancakes? Maybe just adapt it to a regular baking powder recipe?

1 cup unbleached all-purpose flour
1 cup yellow cornmeal
3/4 tsp salt
1 tsp baking soda
1 1/2 tsps cinnamon
1 cup sourdough starter, ripe (fed) or discard
1/4 cup milk
1 large egg
4 Tbsps melted butter or 1/4 cup vegetable oil
1/2 cup, maple syrup, molasses or honey
2 cups blueberries
OPTIONAL : Coarse sugar, for sprinkling tops

1. Preheat the oven to 425°F. Grease enough cups for 12 muffins.
2. Combine the dry ingredients in a mixing bowl.
3. In a second bowl, beat together the starter, milk, egg, melted butter, and sweetener. 
4. Combine the wet ingredients with the dry until the batter is uniform. 
5. Gently stir in the blueberries just until well distributed.
6. Fill the oiled cups 2/3 full; sprinkle optional coarse sugar on top.
7. Bake for 25 minutes, or until a toothpick inserted in the center comes out clean. Remove the muffins from the oven and allow the muffins to cool for 5 minutes before removing them from their cups. 

NOTE: Don't let them cool in their cups, or they'll steam and the outside will become tough.