In a large bowl mix together the pumpkin puree, evaporated milk, brown sugar, eggs, pumpkin pie spice and vanilla extract. Make sure to mix until trace of egg yolk is left behind.
Pour the pumpkin custard mixture into the prepared pan then sprinkle the box of cake mix evenly on top. Then scatter the chopped nuts and add the brown butter.
Bake for 45-50 minutes, until the cake is fully cooked through, if you jiggle the pan the cake should be set. Let cool for at least 15 minutes, you can serve it warm or cool then chill it.