Preheat your oven to 375°F (190°C) and line your muffin tin with paper liners.
Peel, core, and dice the apples.
In a medium-sized bowl, whisk the flour, baking powder, baking soda, salt, and cinnamon until well combined.
In a separate large bowl, cream the butter, brown sugar, and granulated sugar until light and fluffy using a handheld or stand mixer fitted with a paddle attachment.
Add eggs, sour cream, and vanilla and whisk until well combined.
Add the dry ingredients to the wet ingredients in two parts and mix until just combined.
Next, gently fold in the diced apples. Do not overmix.
Divide the batter evenly among the muffin cups.
Make the streusel by combining the brown sugar, flour, cinnamon, and softened butter with a fork. Avoid overmixing, and try to form large crumbles if possible.
Sprinkle evenly on top of the muffin batter.
Bake for 20-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove from the oven and let the muffins cool in the pan for 5-10 minutes before transferring to a wire rack to cool completely.