Preheat the oven to 425°F. Grease or line two standard muffin pans with paper liners.
In a large mixing bowl, whisk together 3 cups of all-purpose flour, 1 ½ tablespoons baking powder, 2 teaspoons cinnamon, ¼ teaspoon nutmeg, and 1 teaspoon kosher salt until evenly combined.
In a separate large bowl, whisk together 1 cup melted butter, ¾ cup granulated sugar, and ½ cup brown sugar. Add 3 eggs and whisk until combined. Add 1 cup sour cream and 1 teaspoon vanilla extract, whisking until smooth.
Add half of the dry ingredient mixture to the wet ingredients, alternating with ½ cup of apple cider. Gently fold together until just combined. Add the remaining dry ingredients and remaining ½ cup apple cider, folding gently until incorporated. Do not overmix! A few small lumps or flour streaks are okay.
Let the batter rest for 10–15 minutes to allow the flour to absorb the moisture.
Divide the batter evenly among the prepared muffin wells, filling each approximately ¾ full.
Bake at 425°F for 5 minutes. Without opening the oven door, reduce the temperature to 350°F and continue baking for 15–18 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
While the muffins bake, combine ¾ cup granulated sugar and 2 teaspoons cinnamon in a shallow bowl. Mix until evenly combined.
Allow the muffins to cool just enough to handle. Brush each muffin generously with 4–6 tablespoons melted butter, covering the tops and sides. Roll or dip in the cinnamon sugar mixture until generously coated.
Serve warm and enjoy!