Preheat the oven to 350°F (175°C). Lightly grease a 9 × 13-inch baking dish.
In a large bowl, whisk together the pumpkin puree, evaporated milk, sugar, egg, vanilla extract, and pumpkin pie spice until smooth and thoroughly combined.
Pour the pumpkin mixture into the prepared baking dish and spread it evenly.
Sprinkle the dry cake mix evenly over the pumpkin mixture. Do not stir.
Drizzle the melted butter evenly over the cake mix. Sprinkle the chopped walnuts or pecans over the top.
Bake for 45–55 minutes, until the top is golden brown and the pumpkin layer is set.
The centre may remain slightly soft but should not look liquid.
Let it cool for approximately 15 minutes before serving.
Serve warm on its own or with vanilla ice cream, whipped cream, or Greek yogurt.