Preheat oven to 350 degrees
Beat together 1 3/4 cups sugar along with the butter and vanilla in a mixer at medium speed until well blended
Add pumpkin puree and eggs one at a time and mix until blended.
In small bowl, whisk together flour, baking powder and salt.
Add flour mixture to sugar mixture and beat at low speed until well blended.
Combine remaining 1/4 cup sugar and cinnamon in small bowl.
Toss the chopped and peeled apples with 2 Tbsp of the cinnamon sugar.
Fold 3 cups of the chopped apples into the cake mixture. Set aside the remaining cup.
Pour batter into a well-greased 8 inch spring form pan.
Add the remaining apples over the batter, evenly, and lightly sprinkle the rest of the cinnamon sugar on top of the batter.
Bake at 350 degrees for 1 hour or until cake slightly browns on top.
Cool on wire rack, then carefully remove spring form pan and serve.
Drizzle with caramel sauce, if desired.