Step 1: Prepare Your Equipment

1. Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease it lightly with nonstick spray.
2. Peel, core, and dice the apples into small, even pieces. Set aside.

Step 2: Mix the Dry Ingredients

1. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. This ensures the spices and leavening agents are evenly distributed.

Step 3: Combine the Wet Ingredients

1. In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth.
2. Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract and yogurt (or buttermilk) until the mixture is fully combined.

Step 4: Fold in the Dry Ingredients and Apples

1. Gradually add the dry ingredients to the wet ingredients, stirring gently with a spatula or wooden spoon. Be careful not to overmix—stop as soon as no streaks of flour remain.
2. Gently fold in the diced apples, ensuring they’re evenly distributed throughout the batter.

Step 5: Fill the Muffin Tin

1. Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full.

Step 6: Prepare the Topping

1. In a small bowl, mix the sugar and cinnamon for the topping. Brush the tops of the muffin batter with the melted butter, then sprinkle generously with the cinnamon-sugar mixture.

Step 7: Bake

1. Bake the muffins in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
2. Remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.