Preheat Oven and Line Muffin Tin:
Heat oven to 350°F (175°C). Place paper liners in a muffin tin.Whisk Dry Ingredients:
In a large bowl, combine flour, granulated sugar, baking powder, salt, and 1 tsp cinnamon by whisking.Mix Wet Ingredients:
In another bowl, whisk eggs, milk, melted butter, and vanilla extract until uniform.Combine Wet and Dry Mixtures:
Add the wet mixture to the dry mixture, stirring gently until barely combined; avoid overmixing.Make Cinnamon-Sugar Filling:
In a small bowl, stir together brown sugar and 2 tsp cinnamon to create the filling.Fill Muffin Cups and Swirl:
Evenly distribute the batter among the muffin cups. Top each with about 1 tsp of the cinnamon-sugar mixture, then gently swirl using a fork.Add Chopped Nuts:
If desired, scatter the chopped nuts evenly over the tops.Bake Muffins:
Bake for 18-20 minutes, until golden and a toothpick inserted in the center comes out clean.Cool Muffins:
Take the pan out of the oven, let the muffins rest for 5 minutes, then move them to a wire rack to cool fully.