unsalted butter, at room temperature; half can be melted and then combined with the unmelted half • light brown sugar • Kosher salt (1/2 teaspoon fine salt) • maple syrup (robust type, if possible) • large egg • vanilla extract • maple extract, if available (otherwise, vanilla type will do) • all-purpose flour • baking soda • maple sugar or maple flakes (if not available, use maple syrup) • (about 7 ounces) walnuts, toasted and chopped, divided