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 and beat for another 2 minutes.
Pour batter into the slow cooker; cover and cook on HIGH for 1 hour 45 minutes or until a toothpick inserted in the center comes out clean.
Uncover and cool for 10 minutes. Run a knife around the edge to loosen the cake; turn out onto a cooling rack and cool completely.
To make the frosting, beat cream cheese, butter, and vanilla with an electric mixer on low speed until smooth.
Gradually add confectioners sugar 1 cup at a time with a hand held mixer on low until smooth and spreadable.
Spread frosting over the cooled cake.
Cut the cake into pieces to serve.
Notes
This slow-cooker pumpkin cake is perfect for fall gatherings and holidays. Store leftovers covered in the refrigerator.