Preheat oven to 325 degrees F. In a 9-inch cast iron skillet melt 1 Tbsp. of the butter over medium-high heat, swirling to coat skillet. Add apple slices; cook 6 minutes or just until light brown, turning once. Remove apples. Cool and wipe out skillet, removing any brown bits.
In a small bowl combine next five ingredients (through salt). In a medium bowl beat 4 Tbsp. of the butter and the brown sugar with a mixer on medium until light and fluffy. Beat in egg and ginger. Gradually beat in molasses. Add flour mixture and buttermilk alternately, beating on low after each addition just until combined. Beat on high 20 seconds more.
Place remaining 1 Tbsp. butter in same skillet. Place skillet in oven until butter is melted; swirl to coat bottom and sides of skillet (if desired, use a silicone pastry brush). Spread batter into prepared skillet. Top with cooked apple slices.
Bake 40 minutes or until a toothpick comes out clean (center may dip slightly). Cool in skillet on a wire rack 30 to 45 minutes. If desired, top servings with yogurt and/or additional apple pie spice.