Preheat the oven to 400F. Grease a rimmed baking sheet on the bottom and sides with butter or spray.
In a large microwavable bowl, add the butter and microwave in 15 second increments until just melted. Whisk in the milk, eggs, maple, and vanilla until smooth.
Add the flours, baking powder, baking soda, and salt and use a rubber spatula to mix until no dry flour remains. It may be a bit lumpy, but that’s normal!
Pour into your prepared sheet pan and use your rubber spatula to carefully spread it into an even layer, then give the pan a shake to even it out.
Top with the blueberries (or whatever add-ins you want). Bake for 12-15 minutes, or until the top is very light golden, then turn your broiler on high and place it on the top rack to broil for about 2 minutes, or until golden brown in spots (keep an eye on it, you may want to rotate halfway through to get more even browning).
Let it cool off for about 5-10 minutes, then slice it up and serve topped with your desired toppings.