Preheat oven to 375 degrees. Spray a standard-sized muffin tin with cooking spray (or use cupcake liners) and set aside. In a mixing bowl, whisk together flour, cornstarch, baking powder, and salt. Set aside.
In a standing mixer, mix together both sugars and butter on high until creamy, about 4-5 minutes. Scrape down sides of the bowl to make sure it's evenly mixed throughout. Add eggs and mix on medium high until well incorporated. Add buttermilk, almond extract, vanilla extract, lemon juice, and lemon zest on medium high speed until well mixed.
Add flour mixture to the standing mixer and stir on the lowest setting until there are no flour pockets. Don't overmix.
Remove the 1 ½ cup of pre-frozen fresh blueberries from the freezer and mix together with 1 teaspoon of flour**. Add to the muffin batter and fold in, being gentle with the blueberries so that they don't burst.
Evenly distribute muffin batter between the 12 muffin tins. Top with 2-3 blueberries (not covered in flour). Bake for 21-25 minutes (until a toothpick can come out clean). Rotate the pan 180 degrees halfway through baking.
Remove muffins immediately from pan and allow to cool.
In a small bowl, whisk together the ingredients for the lemon glaze. Drizzle over the top of the muffins. Enjoy!