Preheat oven to 350°F (175°C). Grease or line a 9×13-inch baking pan 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 granulated sugar until light and fluffy. Add eggs, vanilla extract, and maple extract.
Mix in sour cream and milk until smooth. Gradually add dry ingredients and mix just until combined.
Spread batter evenly in the prepared pan.
Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean. Let cool completely.
To make the glaze, whisk powdered sugar, melted butter, maple syrup, and maple extract. Add milk slowly until pourable.
Spread glaze evenly over cooled bars. Allow to set before slicing.
These Soft Baked Maple Donut Bars bake up fluffy and tender every time.