For the Muffins:

1. Preheat the Oven: Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.

2. Mix Dry Ingredients: In a large bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon.

3. Combine Wet Ingredients: In a separate bowl, whisk together the melted butter, egg, milk, and vanilla extract.

4. Make the Muffin Batter: Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix, as this can make the muffins dense.

5. Fill the Muffin Tin: Divide the batter evenly among the muffin cups, filling each about 3/4 of the way full.

6. Bake the Muffins: Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Remove from the oven and allow to cool slightly.

For the Cinnamon-Sugar Coating:

1. Prepare the Coating: While the muffins are baking, mix the granulated sugar and cinnamon in a small bowl.

2. Brush the Muffins with Butter: Once the muffins are done baking and have cooled for about 5 minutes, brush the tops of each muffin with the melted butter.

3. Coat with Cinnamon-Sugar: Immediately dip the muffin tops into the cinnamon-sugar mixture, coating them generously.

4. Serve and Enjoy: Serve the muffins warm for the best flavor, and enjoy the doughnut-like goodness!