First, preheat oven to 400 degrees F.
In a bowl combine flour, baking powder, baking soda, cornstarch and salt. Set aside.
In a separate bowl beat together butter and sugar until light and fluffy. Add eggs one at a time, scraping down the bowl after each egg, making sure they are combined well. Add lemon zest and mix until combined.
Add half the dry ingredients (flour mixture) to the wet ingredients (sugar, butter and eggs mixture), stirring by hand until just combined.
Next add in half the buttermilk and lemon juice, stir until just combined.
Then add in remaining flour mixture, stirring until just combined. Follow with remaining buttermilk, stirring until just combined.
Fold in blueberries, mixing until just combined. Do not over-mix or as a result muffins may be tough. Batter will be thick and may be a bit lumpy.
Scoop batter by quarter cups into a 12 count muffin pan that has been lined with paper liners or greased. Each muffin cup will be quite full.
Bake for approximately 20 minutes or until a toothpick inserted into the center comes out clean and tops are starting to turn golden brown.
Remove from oven and place muffin pan on a wire rack. Remove from muffin pan after 10 minutes and allow muffins to continue cooling on the wire rack, which will reduce condensation which may occur when cooling directly on a surface.
Finally, if topping with lemon glaze, mix powdered sugar and lemon juice until well combined. Glaze can be as thick or as thin as you like. Add more lemon juice for a thinner glaze or more powdered sugar for a thicker glaze. Drizzle glaze over muffins. Muffins can be glazed hot, which will allow the glaze to soak in, or can be glazed once completely cooled. Enjoy!