In a medium bowl, combine the flour, brown sugar, baking powder, salt and cinnamon. In another bowl, combine the milk, maple syrup, melted butter, sour cream, egg yolk and vanilla.
Stir wet ingredients into dry ingredients.Fold in the chopped walnuts.
Fill a greased 10-ounce ramekin or other similar sized baking dish with the batter. Set aside and make the topping.
TO MAKE THE TOPPING
Combine flour, sugar, cinnamon and nuts; cut in the butter until crumbly. Sprinkle over batter.
Bake for 15 minutes or until a toothpick inserted in center comes out clean. Cool 5 minutes on a wire rack. Enjoy warm.
Notes
Measure Your Ramekin: If your ramekin doesn't have the size indicated, simply fill it with water and then pour that water into a measuring cup to determine its volume.
Mini Loaf Pan Alternative: Don't have a ramekin? No worries, you can also use mini loaf pans that hold between 8 to 10 ounces.
Smaller Ramekin Solution: If you only have smaller ramekins, go ahead and divide the batter between them for smaller muffins.
Muffin Size: This recipe is designed to make one generously sized maple walnut muffin, perfect for satisfying that muffin craving.