Turn your oven on to 425°F so it's fully hot by the time your batter is ready. While it heats, generously coat your 13×18-inch baking sheet with cooking spray or a thin layer of butter, making sure to get into the corners so nothing sticks once it's baked.
In a large bowl, I whisk together the flour, sugar, baking powder, and salt until they're evenly combined. This step only takes a minute, but it keeps the baking powder from clumping in one spot, which can leave you with an uneven rise in the finished pancake.
I make a well in the center of the dry mix and pour in the milk, eggs, melted butter, and vanilla extract. Whisking from the center outward, I slowly bring the flour in until it's just combined. A few small lumps are completely fine here.
Once the batter comes together, I pour it directly onto my prepared baking sheet and use a spatula to spread it evenly to the edges. Getting an even layer here matters because it helps every square bake at the same rate.
This is the fun part for my kids. I sprinkle chocolate chips into one section, blueberries into another, strawberries into the next, and bananas into the last, so everyone gets their favorite pancake without any arguing over toppings.
I slide the pan into the oven and bake for 15 to 17 minutes, watching for a golden top and a center that springs back when touched. Every oven runs a little differently, so I start checking around the 14-minute mark.
As soon as it comes out of the oven, I cut the pancake into 12 even squares using a sharp knife or pizza cutter. I serve it warm with maple syrup right away, while the toppings are still soft and gooey.