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.