In a large bowl, whisk together the angel food cake mix and the pumpkin pie spice. Fold in pumpkin puree and water until well combined. Pour batter into a 9x13-inch baking pan. (Do not grease pan)
Bake for 25-30 minutes, or until the edges of the cake are slightly golden brown and the centers springs back to the touch. Allow to cool completely on a wire rack.
FROSTING: Using a hand mixer, cream together the butter and cream cheese until light and fluffy. Add vanilla. Slowly beat in confectioners' sugar and cinnamon. Continue to beat until everything is well combined. You may need to add a splash of milk, if the frosting is too stiff.
You can cut the cake into bars and pipe the frosting onto each piece or spread the frosting over the entire cake. Refrigerate any leftovers for up to 3 days.