Preheat oven to 350 degrees and line a muffin pan with liners, or grease, and set aside.
In a large bowl, whisk together flour, cinnamon, baking powder, and salt. Set aside.
In a separate bowl, add sugar, melted butter, eggs, milk, and vanilla extract. Mix until fully incorporated. Add to dry ingredients and stir until just combined. Add walnuts and stir to incorporate. Distribute batter between 12 muffin cups.
In a small bowl, combine granulated sugar and cinnamon for the topping. Sprinkle evenly over the muffins.
Bake for 18-22 minutes, or until a toothpick inserted in the middle comes out clean (or with crumbs, not batter).
Allow pan to cool for 2-3 minutes before removing muffins to a cooling rack to cool completely. Store cooled muffins in a sealed container.
Muffins will keep at room temperature in a sealed container for 3 days, or up to a week in the refrigerator.
Baked muffins can be stored in the freezer for up to 3 months, tightly wrapped in foil and stored in a freezer safe container.
If using paper liners, allow your muffins to cool completely before unwrapping, or they will stick to the papers.