Preheat oven to 375F (190C) and line a 12-count standard muffin tin or 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 in the melted butter until incorporated (mixture may curdle slightly, this is fine).

Add the 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 15-16 minutes (for 12 standard muffins) or 11-13 minutes (for mini muffins) and 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 topping while muffins are still warm.

Brush melted butter over each muffin while still warm.

Stir together sugar and ground cinnamon and then sprinkle over each muffin.

Serve warm and enjoy!