Cream butter and sugar together for 2-3 minutes and then add eggs and beat for another 1-2 minutes.
Add pumpkin and vanilla and mix well. Add flour, pumpkin pie spice and cinnamon and mix until well combined.
Pour batter into a well greased 10X15X1 pan. Bake for about 20-25 minutes or until a toothpick inserted in the center comes out clean. Let cool before making the frosting.
To make the frosting, beat the cream cheese, vanilla and butter together until light and fluffy. Add 2 cups of the powdered sugar and mix well. Add the other 2 cups of powdered sugar and mix until smooth. If the frosting is a little bit too thick to spread, you can add 1-2 tablespoons of milk to thin it out. Spread over the top of the cooled brownies. Add sprinkles if desired. Slice and serve.
You can store these at room temperature or in the refrigerator. My kids actually prefer them cold, but it's totally up to you!