Preheat oven to 350 degrees. Spray a 13 by 9-inch baking dish with non-stick cooking spray.
In a large mixing bowl whisk together pumpkin, half and half, eggs, vanilla extract, brown sugar, pumpkin pie spice, and salt. Whisk several minutes until well combined.
Pour pumpkin mixture into prepared baking dish then wiggle pan to level the mixture out even.
Gently rub the cake mix package to break up any clumps then cut one corner from the package and gently pour mixture evenly over pumpkin mixture layer in pan.
Using your hand gently spread the top surface of cake mix out even (being careful not to submerge it into the pumpkin mixture).
Evenly sprinkle pecan pieces over cake mix layer then evenly dot with butter pieces.
Bake in oven until pumpkin filling is nearly set, the cake should just wiggle slightly, about 50 to 60 minutes.
Transfer to wire rack to cool. You can serve warm (which is messier but delicious), or cool for 1 hour before cutting into cleaner slices. These can also be served cold from the fridge.
If desired served with sweetened whipped cream, vanilla ice cream or salted caramel sauce on top.
