Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or spray with non-stick cooking spray.
Peel, core, and dice the apples into small, uniform ¼-inch cubes.
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt.
In a large bowl, whisk together the melted butter and sugar until well combined. Add eggs one at a time, beating well after each addition. Stir in the milk and vanilla extract.
Add the dry ingredients to the wet ingredients and stir just until combined. Do not overmix - a few lumps are fine.
Gently fold in the diced apples, distributing them evenly throughout the batter.
In a small bowl, combine the brown sugar, flour, and cinnamon for the streusel. Cut in the cold butter using a pastry cutter or your fingertips until the mixture resembles coarse crumbs. Add chopped nuts if desired.
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full. Sprinkle the streusel topping over each muffin.
Bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean. For perfectly even baking, rotate your muffin tin halfway through the baking time.
Allow to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.