Preheat your oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease.

In a large mixing bowl, whisk together the dry ingredients: flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

Add the sugars to the dry mixture and whisk until fully incorporated.

In a separate bowl, whisk together the wet ingredients: melted butter, eggs, buttermilk, and vanilla until smooth.

Pour the wet ingredients into the dry ingredients and gently fold until just combined, with some flour streaks remaining.

Fold in the diced apples and nuts until evenly distributed.

Fill each muffin cup about two-thirds full with the batter.

If desired, mix together a tablespoon of sugar and a teaspoon of cinnamon, and sprinkle it over the muffin batter.

Bake for 18-20 minutes until golden brown and a toothpick comes out clean.

Allow to cool in the pan for 5-10 minutes, then transfer to a wire rack to cool completely.