Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish to prevent sticking.
In a large bowl, whisk together pumpkin puree, eggs, sugar, cinnamon, nutmeg, and vanilla until smooth.
Spread the pumpkin mixture evenly into the prepared baking dish. Sprinkle the dry yellow cake mix evenly over the top—do not stir.
Scatter the chopped nuts across the surface, then drizzle evenly with melted butter.
Bake for 45–50 minutes, or until the top is golden brown and slightly bubbly around the edges.
Let the cake cool slightly before serving. Pair with whipped cream or a scoop of vanilla ice cream for the perfect finishing touch.