Preheat your oven to 180°C (350°F). Grease the muffin tin or line with paper liners, and set aside.
In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and ground cinnamon. This creates an even, dry base for your batter.
In a separate bowl, whisk the egg, milk, and melted butter until smooth and slightly glossy. This helps the wet ingredients combine evenly and relax the batter.
Pour the wet mixture into the dry ingredients. Gently fold with a spatula just until combined—don’t overmix, a few lumps are okay. The batter should be thick but pourable.
Spoon the batter into the prepared muffin tin, filling each cup about two-thirds full. This leaves room for them to puff up nicely in the oven.
In a small bowl, mix the sugar and cinnamon for the topping. Sprinkle generously over each muffin, creating a crackly, fragrant crust.
Bake in the oven for 20-22 minutes, or until the muffins are golden on top and a toothpick inserted in the center comes out clean. The tops will crack slightly, releasing a warm cinnamon aroma.
Remove the muffins from the oven and let them cool in the tin for about 5 minutes. Then, transfer to a wire rack to cool slightly and set the crust.
Serve the muffins warm with the crackling cinnamon-sugar crust still crisp, or at room temperature. They’re best enjoyed within a couple of days for maximum freshness.