Preheat oven to 350 degrees F (180 degrees C). Line muffin tin with paper cups.
Whisk the All-Purpose Flour (1 1/2 cup), Baking Powder (1 1/2 teaspoon) and Salt (1/2 teaspoon) in a bowl.
Cream the Butter (5 tablespoon) with the Pure Maple Syrup (1/2 cup) for about 2 minutes.
Add Egg (2), one at a time, beating well after each addition.
Add the flour mixture (in three increments) alternately with the Milk (3/4 cup) (in two increments) beginning and ending with flour.
Fold in the Walnut (1/2 cup).
Fill muffins cups 3/4 full. Bake for 20 minutes, then let cool on a wire rack.
Make the optional maple icing by combining Butter (4 1/2 teaspoon), Maple Syrup (1 1/2 tablespoon), and Milk (1 tablespoon) in a saucepan and heat until melted.
Remove from heat and whisk in Powdered Confectioners Sugar (1/2 cup) and Salt (1 pinch). Let cool before drizzling over tops of cooled muffins.