Preheat oven to 350°F. Grease a 9x13 inch pan or line it with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
In a large bowl, beat butter, brown sugar, and sugar until light and fluffy. Add eggs, vanilla, and maple extract.
Mix in sour cream and milk. Gradually add dry ingredients until combined.
Pour batter into the prepared pan and spread evenly.
Bake for 20 to 25 minutes or until a toothpick comes out clean. Let cool completely before glazing.
To make the glaze: Whisk powdered sugar, melted butter, maple syrup, and maple extract in a bowl.
Add milk slowly until smooth and pourable.
Spread glaze evenly over cooled bars. Allow it to set before cutting into squares or rectangles.
