Add butter and sugar to a 10 inch cast iron pan. Cook over low until they melt.
Add apples. Cook over medium-low for 10 minutes to soften.
Meanwhile, cream together the butter and sugar until light and fluffy. Mix in the vanilla extract and the eggs.
Add the sour cream and mix well.
Add the dry ingredients and mix until thoroughly incorporated.
Remove the cast iron pan from the heat. Spoon the cake batter over the top of the apples.
Spread gently so the batter is even.
Place on a baking pan lined with foil (in case the juices overflow) and bake for 25 -30 minutes.
Meanwhile, mix together the streusel ingredients to form large crumbs. Spread on a rimmed baking pan. Bake at the same time as the cake for 8 minutes or until the crumbs are golden.
Allow the cake to rest in skillet for 5 minutes, then invert onto a serving plate.