Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners. Whisk together flour, sugar, baking powder, salt, and cinnamon in one bowl. In another, combine melted butter, milk, eggs, nutmeg, and vanilla extract until smooth.
Gently fold the dry mixture into the wet mixture in three additions. Stir just until combined to avoid overmixing, which can make the muffins dense. The batter should be thick but pourable.
Spoon the batter into muffin cups, filling each about two-thirds full. Bake for 20–25 minutes until golden brown and a toothpick comes out clean. Let them cool in the tin for 5 minutes before transferring to a wire rack.
Mix sugar and cinnamon in a shallow dish. While muffins are still warm, dip the tops into the mixture, pressing gently to create a sweet, crunchy layer. For extra crispiness, pop them back in the oven for 2–3 minutes.