Preheat oven to 350°F (175°C). Line a 9x9 inch pan with parchment paper, leaving overhang on sides.
In a medium bowl, whisk together flour, baking soda, baking powder, salt, and nutmeg. Set aside.
In a large bowl, cream butter and sugar until light and fluffy, about 2 minutes. Add egg and vanilla, mix well.
Add buttermilk to the butter mixture and stir gently until combined.
Add dry ingredients to wet mixture in two batches, stirring just until combined. Do not overmix.
Spread batter evenly in prepared pan using a spatula.
Bake for 20-22 minutes until a toothpick inserted in center comes out with a few moist crumbs.
Cool in pan for 10 minutes, then lift out using parchment paper and transfer to a wire rack to cool completely.
For glaze, whisk together powdered sugar, maple syrup, maple extract, and milk until smooth and pourable.
Pour glaze over cooled bars and spread evenly. Let set for 15 minutes before cutting into 12 squares.