Melt 2 tbsp butter in a cast iron skillet. Once melted and bubbling (slightly brown), add in apples. Saute for 5 minutes, until slightly broken down.
Remove skillet from heat and stir in caramel topping. Combine walnuts with caramel mixture.
In a large bowl, mix together spice cake mix, melted stick of butter, milk, and pumpkin puree. Pour over caramel mixture in skillet (do not mix).
Bake at 350 degrees for about 40 minutes, until toothpick inserted halfway into cake comes out clean.
Turn cake onto a cake pan, apple side facing up.