In a large mixing bowl, beat butter and sugar until light and fluffy.
Add eggs, one at a time, beating well after each addition.
Beat in pumpkin.
In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.
Add flour mixture to the mixing bowl alternately with buttermilk, beating to combine after each addition.
Spread cake batter carefully over apple slices.
Bake at 350 degrees for 30-35 minutes, or until a toothpick inserted near the center comes out clean.
Using a knife, loosen the sides of the cake from the pie plate so they don't stick as it cools.
Cool for 15 minutes before inverting onto a serving plate.
Serve warm with ice cream!