Preheat oven to 350 degrees Fahrenheit. Spray a 9x13 inch baking pan generously with nonstick spray.
In a large bowl, whisk pumpkin puree, evaporated milk, brown sugar, eggs, and pumpkin pie spice until smooth. Pour into prepared pan and spread evenly.
Sprinkle the entire box of cake mix evenly over the pumpkin layer. Do not stir.
Scatter crushed graham crackers (or pecans) and toffee bits over the cake mix.
Drizzle melted butter evenly over the entire surface, making sure to cover all the dry cake mix.
Bake 45 to 50 minutes until center is set and edges are lightly golden.
Serve warm or at room temperature with ice cream or whipped