Heat oven to 350°F (175°C). Coat a 9x13-inch baking pan with grease.
In a big bowl, whisk together pumpkin puree, sugar, pumpkin spice seasoning, eggs, evaporated milk, and vanilla extract until smooth.
Transfer the pumpkin blend into the greased pan. Evenly scatter the dry cake mix on top.
Evenly distribute the chopped pecans over the cake mix layer.
Cut the butter into thin slices and lay them evenly across the top of the mixture.
Bake at 350°F (175°C) for 1 hour, until set and golden brown.
Let cool briefly before serving. Serve warm with optional ice cream or whipped cream.