Preheat the oven to 350 degrees, and generously grease a 9x13 inch pan. Set aside.
In a large bowl, whisk together the melted butter, brown sugar, and regular sugar until smooth. Whisk in the eggs, milk, vanilla extract, and maple extract until the mixture is smooth. Add the flour and salt to the mixture, and stir until combined. Gently stir in the chocolate chunks and walnuts, working quickly because the batter will be starting to thicken as the butter comes back to room temperature.
Spread evenly in your prepared pan. Bake for 28-30 minutes, or until a toothpick inserting about 2 inches from the edge of the pan comes out clean. Cool for 15-20 minutes before icing.
With a whisk, stir all the ingredients together in a bowl until smooth. Add more water to reach a consistency you can drizzle. Generously drizzle over the bars. Enjoy!