Line an 8×8-inch pan with foil and grease with butter. Set aside.
In a large pot, combine brown sugar, evaporated milk, and butter. Bring to a boil over medium heat, stirring until butter melts.
Reduce heat to medium-low and boil for 10 minutes, stirring constantly.
Remove from heat. Whisk in powdered sugar, vanilla extract, and maple extract until smooth.
Pour into a stand mixer bowl fitted with paddle attachment. Beat on low for 30 seconds, then medium for 2 minutes until glossy and thick.
Stir in chopped walnuts by hand.
Pour into prepared pan and spread evenly. Cool to room temperature, then cover and refrigerate overnight (at least 6 hours).
Lift out by the foil overhang, place on a cutting board, and cut into squares with a warm, dry knife.