Preheat the oven to 425 degrees. Spray a rimmed 13 x 18 inch sheet pan with cooking spray (this will keep the parchment in place.) Cut a piece of parchment paper to cover the bottom completely. Place on a sheet pan and spray more oil on the parchment and around the sides of the sheet pan.
In a medium bowl, whisk together the flours, sugar, baking powder, baking soda, and salt. In another medium bowl, whisk together the butter, buttermilk, eggs, lemon juice and zest, vanilla, and 1/4 cup water until thoroughly combined.
Add the wet ingredients to the dry ingredients and whisk until just combined. Do not overmix (or worry if there are some lumps).
Gently fold in the blueberries with a spatula.
Pour the batter into the prepared sheet pan. Spread evenly with a spatula, then tap the sheet pan on the counter a few times to settle the batter. Bake, rotating the pan halfway through, until golden and set, about 15 minutes.
Let cool for 5 minutes in the pan. Place a large cutting board over the top of the pan and flip the pancake onto a cutting board. Cut into 16 squares. Add toppings as desired, and serve.
Meal Prep: Cool completely before transferring to an airtight container. To reheat from the fridge, microwave for 40 seconds, flipping halfway through. To reheat from frozen, microwave for 45 seconds. Flip and cook for an additional 45 seconds, or until heated through.