Preheat your oven to 350°F (175°C) and grease a 12-cup muffin tin or line it with muffin liners.
In a small saucepan, bring the apple cider to a boil over medium-high heat. Reduce to a simmer and let it cook for about 10 minutes, or until reduced to 1/2 cup. Remove from heat and set aside to cool slightly.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt until well combined.
In another bowl, whisk together the melted butter, brown sugar, and the reduced apple cider until smooth. Stir in the egg, vanilla extract, and buttermilk until fully incorporated.
Gradually add the wet mixture to the dry ingredients, stirring gently until just combined. Don’t overmix; a few lumps are perfectly fine.
Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake in the preheated oven for about 18-20 minutes or until a toothpick inserted into the center comes out clean.
While the muffins are baking, mix together the granulated sugar and cinnamon in a small bowl for the dusting.
Once done, remove the muffins from the oven and let them cool in the tin for about 5 minutes. Afterward, remove them from the tin and roll each warm muffin in the cinnamon sugar mixture until fully coated.