Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease the cups with non-stick spray.
In a medium bowl, whisk together all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, 1/2 teaspoon ground cinnamon, and salt until well combined.
In a separate bowl, whisk the milk, vegetable oil, egg, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula until just combined with a few lumps remaining; do not overmix.
Spoon the batter evenly into the prepared muffin cups, filling each about two-thirds full.
Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean and tops are lightly golden and spring back when touched.
While the muffins bake, combine 1/4 cup granulated sugar and 1 teaspoon ground cinnamon in a small bowl; set aside.
Once muffins are out of the oven, let cool in the pan for 5 minutes, then transfer to a cooling rack for 5 minutes. While still warm, dip each muffin in melted butter, then roll in the cinnamon sugar mixture until fully coated.
Serve warm or at room temperature, perfect with coffee or tea.