Lightly coat a 6-quart slow cooker with nonstick cooking spray. For easier serving and removal, add a parchment-paper liner if desired.

Whisk the pumpkin purée, evaporated milk, eggs, granulated sugar, cinnamon, and nutmeg in a large bowl until smooth and evenly blended.

Pour the pumpkin mixture into the prepared slow cooker and smooth the surface. Sprinkle the dry yellow cake mix evenly over the pumpkin layer, but do not stir.

Drizzle the melted butter over the cake mix, distributing it as evenly as possible so there are no large dry patches. Scatter the chopped pecans over the top if using.

Cover and cook on HIGH for 2 to 3 hours or on LOW for about 4 to 5 hours. The edges should be set and the center should be mostly firm while remaining moist.

Turn off the slow cooker and let the cake stand, covered, for 15 to 20 minutes before serving. Spoon it into bowls and serve warm with whipped cream or vanilla ice cream.