Preheat oven to 400°F and lightly coat a muffin tin with baking spray, or line with paper liners.

In a large bowl, mix together flour, baking powder, and salt. Set aside.

In another bowl, cream together butter and brown sugar. Beat in egg, buttermilk, maple syrup, and maple extract.

Pour the wet ingredients into the flour mixture and stir until mixed, but do not overmix. It should be slightly lumpy and somewhat thick. Fold in walnuts.

To make the topping, cut together butter, brown sugar, flour, and cinnamon using a pastry blender or two knives, until the mixture forms crumbs.

Divide the batter among the muffin cups and sprinkle on topping.

Bake for 18 minutes, until a toothpick inserted in the center is clean and muffins are golden brown. Let cool slightly and serve.