Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly.

In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon until well combined.

In another bowl, mix the melted butter, milk, eggs, and vanilla extract. Ensure that these ingredients are thoroughly blended.

Pour the wet mixture into the dry mixture. Gently fold until just combined—don’t overmix! A few lumps are fine.

In a small bowl, mix brown sugar and ground cinnamon. Add chopped nuts if you’re using them for an extra crunch.

Spoon half of the muffin batter into the prepared muffin cups. Sprinkle a small amount of the filling mixture on top, then add the remaining batter carefully over that, followed by another sprinkle of the filling on the top.

Bake in the preheated oven for about 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Allow the muffins to cool in the pan for a few minutes before transferring them to wire racks. Enjoy them warm or at room temperature!