1. Preheat the oven to 350°F and line an 8-inch square baking pan with parchment paper.

2. Brown the butter in a saucepan over medium heat until golden, nutty, and fragrant, then transfer it to a bowl and let it cool slightly.

3. Whisk together flour, baking powder, salt, cinnamon, and nutmeg in a medium bowl.

4. In a separate bowl, whisk brown sugar, granulated sugar, eggs, vanilla, sour cream, milk, and cooled brown butter until smooth.

5. Fold the dry ingredients into the wet ingredients until just combined.

6. Spread the batter evenly into the prepared pan.

7. Bake for 24 to 28 minutes, or until the center is set and a toothpick comes out with a few moist crumbs.

8. Whisk powdered sugar, maple syrup, milk, vanilla, and a pinch of salt until smooth.

9. Pour the maple glaze over the slightly warm bars and sprinkle with toffee bits or toasted pecans.

10. Let the glaze set, then slice into bars and serve.