In a large bowl, beat together the butter and brown sugar until creamed. There should be no lumps of butter.
Mix in the maple syrup, egg, vanilla extract and optional maple extract.
Add the flour, baking soda and salt to the bowl. Mix together, starting with the mixer on a low speed. Once finished, if the dough sticks to your fingers when you squeeze it in your hands, mix in 1-2 extra tablespoons of flour.
Stir in the chopped nuts.
Cover the bowl with plastic and place in the fridge for at least 4 hours or overnight.
When ready to bake, preheat the oven to 350F (180C) and line cookie sheets with parchment paper.
Form the dough into balls with about 1 ½ - 2 tablespoons of dough each. Place at least 2 inches apart on the lined cookie sheets. If dough starts to warm up and become sticky, place it back in the fridge. I always place trays of cookie dough balls back in the fridge while I wait for the oven to keep the dough balls cold.
Bake 1 sheet at a time in the middle of the oven for about 11-13 minutes, or until the tops look set.
Remove from the oven and cool on the cookie sheets.