Preheat oven to 350 degrees.
Butter a 9-inch spring-form pan.
Combine flour, 4 Tablespoons butter,1 cup brown sugar and salt in a large bowl.
Cut in the butter, by hand or by mixer, until the mixture resembles coarse meal.
Set aside 2/3 cup of the dry mixture to be used for the streusel.
In a medium size mixing bowl, combine the pumpkin, sour cream, 2 Tablespoons granulated sugar, pumpkin pie spice and baking soda, whisking to combine. Stir the pumpkin mixture into the dry cake ingredients.
Add the eggs, mixing well.
Transfer the batter into the buttered spring form pan.
Scatter the cooled apples evenly over the top.
Sprinkle the reserved crumb topping over the apples.
Place the spring-form pan on a baking sheet or aluminum foil. Bake at 350 degrees until the topping is golden brown and a toothpick inserted into the center of the cake comes out clean – about 1 hour and 10 minutes.
Cool the cake on a rack for 20 minutes. Run a knife around the edge of the spring-form pan to loosen the sides before removing.
Serve the cake warm, with whipped cream, or at room temperature. . .
Enjoy!