Preheat your oven to 425°F (220°C). Generously grease a 18×13-inch rimmed baking sheet with butter or non-stick cooking spray. For even easier removal, you can line the bottom with parchment paper and grease the paper as well.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt. Make sure there are no large lumps of baking powder so the pancakes rise evenly.
In a separate medium bowl, whisk together the milk, eggs, melted butter, maple syrup, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients. Use a spatula or whisk to gently fold the mixture together until just combined. Be careful not to overmix; a few small lumps are perfectly fine and help keep the pancakes fluffy.
Pour the batter onto the prepared sheet pan. Use a spatula to spread the batter evenly into all four corners and smooth the top.
Sprinkle the chopped pecans evenly over the top of the batter. You can gently press them into the batter if you want them more embedded.
Place the pan in the oven and bake for 15 to 20 minutes. The pancakes are done when they are golden brown on top and a toothpick inserted into the center comes out clean.
Remove the pan from the oven and let it cool for a couple of minutes. Use a knife or a pizza cutter to slice the pancakes into squares or rectangles. Serve immediately while warm.