Preheat oven to 400°F. Line a 13-by-18-inch sheet pan with parchment paper.
Whisk together the melted butter, buttermilk or milk, egg, pumpkin puree, and vanilla.
In a separate bowl, whisk together the flour, sugar, baking powder, 1 teaspoon of pumpkin pie spice, and sea salt.
Stir the wet ingredients into the dry ingredients just until combined. Spread the batter evenly into the prepared sheet pan.
In a small bowl, combine the toasted pecans, cane sugar, and remaining 1 ¼ teaspoon pumpkin pie spice. Sprinkle the pecan topping evenly over the batter.
Bake for 18-22 minutes, or until the center springs back when gently pressed.
Allow to cool for a few minutes, then cut into12 squares. Dust with powdered sugar and serve with maple syrup.