Preheat the oven to 350 degrees F. Grease a 9 X 13 inch baking dish and set aside.
In a large bowl, mix the flour, sugar, baking powder, and salt together. Melt 12 tablespoons butter and add to the mixture along with the sour cream, eggs, and vanilla extract. Stir well to combine.
Stir in the diced apple. Then spread half the cake batter in the bottom of the prepared baking dish.
In a medium bowl mix the chopped pecans, brown sugar, and apple pie spice together. Sprinkle half the pecan mixture evenly over the cake batter. Then spread the remaining cake batter over the top.
Add the remaining 4 tablespoon of butter to the remaining pecan mixture. Use your hand to mix into a paste. Drop small dollops of pecan-butter over the surface of the cake.
Bake for 45-50 minutes, until a toothpick inserted into the center of the cake comes out clean. Cool before cutting.