Preheat your oven to 350°F. Grease your muffin pans and line the bottom with circles of parchment paper, to make the cups easier to get out when baked.
In your stand mixer, cream together the butter and sugars until light and fluffy.
Mix the eggs and vanilla into the butter mixture.
In a separate bowl, mix together the flour, oats, baking powder, cinnamon, pie spice, and salt.
Add this dry mixture to the butter and egg mixture, and mix until it comes together.
Scoop about 2 Tbsp into each muffin cup. Then, using a shot glass, press down to squeeze it around into a cup shape. It'll be sticky and a bit messy, but it'll work.
Bake your muffin cups for about 20 minutes. When done, remove and let cool in the pan for 10 minutes, then carefully remove them and put them on a cooling rack to finish cooling.