In a large bowl, combine the all-purpose flour, sugar, cornstarch, baking powder, and salt.
In a small bowl, combine the egg, milk, vanilla extract, and 4 tablespoons of butter, whisking to combine.
Add the wet ingredients to the dry ingredients and mix until combined.
Pour the batter into a sheet pan prepared with parchment paper and greased with butter.
Jiggle the pan slightly to spread the batter evenly across the pan.
Prepare the lemon glaze by combining the powdered sugar, lemon juice, lemon zest, and melted butter in a small bowl, whisking until smooth.
Drizzle half of the lemon glaze over the pancake mix and use a butter knife to create swirls in the batter.
Sprinkle with blueberries and place in the oven.
Bake for 15-20 minutes, checking the middle with a toothpick to confirm doneness.
Top the pancakes with remaining lemon glaze and blueberries, if desired.
Notes
Make Ahead: If you want to get a head start on this recipe, you can prepare the batter and store in an airtight container in the refrigerator overnight. The next morning, simply spread it into a sheet pan and bake according to the instructions below.
Storage: Store leftovers in an airtight container in the refrigerator for up to 5 days, or frozen for up to 3 months. You can place parchment paper between the layers to keep them from sticking.
Reheating: To reheat, place a pancake slice in the microwave for 15-20 seconds, warm it up in a toaster oven, or heat it on a sheet pan in a 325ºF oven. If you’re using the oven, I recommend covering the pancake with foil to keep it from drying out.