Preheat oven to 375F (190C) and line a 24 count mini muffin tin with paper liners or lightly grease and flour.
Combine flour, sugar, baking powder, cornstarch salt, cinnamon, and nutmeg in a large bowl and whisk until well-combined.
In a separate bowl, whisk together egg, vanilla, and buttermilk.
While still whisking the wet ingredients, slowly pour butter until incorporated (mixture may curdle slightly).
Add dry ingredients to wet and, using a spatula, gently stir ingredients together until just combined -- do not over-mix or your muffins will be dense and dry!
Portion muffin batter into prepared mini muffin tin, filling each liner ¾ of the way full.
Transfer to 375F (190C) oven and bake for 11-13 minutes or until a toothpick inserted in the center comes out with moist crumbs (not wet batter, there should be crumbs or the toothpick should be clean, but don't over-bake muffins or they will be dry).
Prepare your topping by brushing butter over each muffin while still warm.
Stir together sugar and ground cinnamon and then sprinkle over each muffin.
Serve warm and enjoy!
*If you don't have buttermilk, you can use my easy buttermilk substitute which would involve pouring ½ Tablespoon (1 ½ teaspoons) of vinegar or lemon juice into a measuring cup then adding milk and filling to the "½ cup" mark. Stir and allow to sit for 5 minutes before using.