Generously spray a 5 1/2- to 6-quart slow cooker with cooking spray.

In a large bowl, beat cake mix, pumpkin, water, vegetable oil, eggs, and pumpkin pie spice with an electric mixer on low speed for 1 minute, scraping the bowl constantly. Increase speed to medium; beat for 2 minutes. (This can also be done by hand with a whisk).

Pour batter into the slow cooker; cover and cook on HIGH for 1 hour and 45 minutes, or until a toothpick inserted in the center comes out clean.

Uncover and cool for 10 minutes. (Make sure the slow cooker is off). Run a knife around the edge to loosen; turn out onto a cooling rack and cool completely.

In a medium bowl, beat cream cheese, butter, and vanilla with an electric mixer on low speed until smooth.

With a hand mixer on low, gradually add in confectioners sugar, 1 cup at a time, until smooth and spreadable.

Spread over the cooled cake.

Cut into pieces to serve.