In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and granulated sugar until evenly combined.

Melt 1/4 cup unsalted butter in the microwave, taking care not to overheat. Allow the melted butter to cool slightly before adding to the batter later.

Create a well in the center of the dry ingredients. Add the cooled melted butter from Step 2, milk, egg, and vanilla extract. Whisk the mixture until it becomes a smooth batter with no lumps.

Place 2 tablespoons of unsalted butter into a 9x13-inch casserole dish and put the dish in the oven as it preheats to 350℉. Let the butter melt fully, which should take a few minutes. Once melted, remove the dish and brush the melted butter evenly over the bottom and sides.

While the butter is melting in the oven, gently toss the fresh blueberries with 1 teaspoon of flour in a bowl so that they are all lightly coated. This helps prevent the berries from sinking during baking. I like this step because it keeps the blueberries evenly distributed in every bite!

Pour the batter from Step 3 into the buttered casserole dish (from Step 4). Evenly sprinkle the flour-coated blueberries (from Step 5) over the batter. Bake in the preheated oven at 350℉ for about 30 minutes, or until golden on top and set in the center.

Remove the baked pancake casserole from the oven. If desired, sprinkle powdered sugar on top before serving. Cut into squares and enjoy with your favorite syrup.