01 - Preheat oven to 350°F. Grease and line an 8x8-inch baking pan with parchment paper.
02 - In a large bowl, whisk together melted butter, brown sugar, and maple syrup until smooth.
03 - Add eggs and vanilla extract to the butter mixture; whisk until fully combined.
04 - Sift flour, baking powder, and salt into the wet ingredients. Stir gently until just combined, avoiding overmixing.
05 - Pour batter into the prepared pan and smooth the top with a spatula.
06 - Bake for 22 to 25 minutes, or until a toothpick inserted in the center comes out clean. Cool completely in the pan on a wire rack.
07 - Whisk powdered sugar, maple syrup, milk starting with 1 tablespoon, and a pinch of salt until smooth and pourable. Add additional milk if needed to achieve desired consistency.
08 - Spread glaze evenly over cooled bars. Allow to set for 15 minutes, then cut into 12 bars.