Combine dry ingredients. In a medium bowl, whisk together flour, baking powder, salt, cinnamon, and nutmeg until well combined.
Cream butter and sugars. In a large bowl, use an electric mixer to beat the butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy, about 2-3 minutes. Be sure to scrape down the sides of the bowl occasionally.
Add wet ingredients. Beat in the egg until fully incorporated, then add maple syrup and vanilla extract, mixing until smooth. The mixture might look slightly curdled – that’s completely normal.
Incorporate dry ingredients. Gradually add the flour mixture to the wet ingredients on low speed, mixing just until combined. Avoid overmixing, which can make the cookies tough.
Chill the dough. Divide the dough in half, form into discs, wrap in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 days).
Prepare for baking. When ready to bake, preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Roll and cut cookies. On a lightly floured surface, roll one disc of dough to about ¼-inch thickness. Use leaf-shaped cookie cutters to cut out shapes, re-rolling scraps as needed.
Bake to golden perfection. Place cookies 1 inch apart on prepared baking sheets and bake for 10-12 minutes until edges are just beginning to turn golden. Look for slight firmness at the edges while centers remain soft.
Cool completely. Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Make the glaze (if using). Whisk together powdered sugar and maple syrup, adding just enough milk to reach a drizzling consistency. If coloring, divide into portions and add food coloring to create fall shades.
Decorate your leaves. Drizzle or spread glaze on cooled cookies, adding details like leaf veins if desired. Let the glaze set completely before storing.