In a large bowl or stand mixer, cream the softened butter and granulated sugar/brown sugar together until the mixture is light and fluffy, usually for about 2-3 minutes.
Preheat your oven to 375°F. Line a muffin tin with paper liners or grease it lightly. In a medium-sized bowl, whisk together the flour, baking powder, salt, ground cinnamon, apple pie spice and ground nutmeg. Set this dry mixture aside.
Add the apples, eggs and vanilla to the wet mix, slowly mix the flour mix into the wet mix. Batter will be a bit thick, do not over mix otherwise the muffins can get ruined with too much air.
Add the muffin mix to the muffin liners. If you are using regular sized muffin liners, fill about 1/2 of the way (this will make over 12 muffins!) If you are using large tulip muffin liner (as pictured) this will make about 9 large muffins.
Make the streusel topping. In a skillet, add the butter, spices, salt, apples, oats and brown sugar. Mix together well for about 1-2 minutes just until the butter is melted and everything is evenly spread and mixed.
Add about 1-2 Tbsp of the streusel to the top of the muffin batter. Gently press down if needed to get the streusel to stick. Place in the oven for 18-20 minutes. *please note all oven temps vary so check that a toothpick comes out clean to ensure it's cooked through.