Spray the insert of a 6-quart slow cooker with cooking spray.
Gently combine the pumpkin puree, granulated sugar, pumpkin pie spice and salt in a bowl, then dump it into the slow cooker insert. Sprinkle the cake mix over the pumpkin puree, then, as evenly as possible, pour the melted butter on top and finish with the chopped pecans. Cover and cook on low for 4 to 4 1/2 hours (the edges of the cake should be starting to brown and crisp slightly).
Meanwhile, using a handheld mixer, whip the cream and confectioners' sugar together in a large bowl until stiff peaks form. Serve the cake warm with a dollop of the whipped cream.