- Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it with non-stick spray. 
- In a large bowl, whisk together the flour, baking powder, salt, ground cinnamon, and granulated sugar until well combined. 
- In another bowl, mix the milk, melted butter, egg, and vanilla extract until smooth. 
- Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay. 
- In a small bowl, combine the brown sugar, ground cinnamon, and chopped nuts (if using). 
- Fill each muffin cup about halfway with the batter, then add a spoonful of the cinnamon-sugar mixture on top. Cover with more batter until the cups are about 3/4 full. 
- Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. 
- While the muffins are baking, prepare the glaze by mixing the powdered sugar with 1-2 teaspoons of milk until smooth. 
- Once the muffins are done, let them cool in the pan for 5 minutes before transferring them to a wire rack. Drizzle the glaze over the warm muffins.