Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan and line it with parchment paper, leaving an overhang on the two long sides for easy removal.
In a large bowl, whisk together the flour, granulated sugar, baking powder, cinnamon, salt, and nutmeg until thoroughly combined and no lumps remain.
In a separate medium bowl or large measuring cup, whisk the milk, eggs, melted butter, oil, and vanilla extract until the mixture is completely smooth and emulsified.
Pour the wet ingredients into the dry ingredients. Using a spatula, gently fold the mixture together until just combined and no dry streaks of flour remain. Be careful not to overmix.
Pour the batter into the prepared pan and spread it into an even layer. Bake for 23-28 minutes, or until the top is lightly golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the bars cool in the pan on a wire rack for 10 minutes. Then, using the parchment overhang, lift the entire slab of bars out of the pan and place it directly on the cooling rack. Let cool completely, about 1 hour.
While the bars cool, make the glaze. In a medium bowl, whisk together the sifted confectioners' sugar, maple syrup, vanilla, and salt. Add milk or cream, one tablespoon at a time, until you reach a thick but pourable consistency.
Once the bars are completely cool, pour the maple glaze evenly over the top, using an offset spatula to spread it to the edges. Immediately sprinkle with maple or coarse sugar if using. Allow the glaze to set for 20-30 minutes before slicing into 12 bars.