How to Make It
Preheat oven to 400°.
Weigh or lightly spoon flours and potato starch into dry measuring cups; level with a knife. Combine flours, potato starch, baking soda, xanthan gum, and salt in a medium bowl, stirring with a whisk.
Place buttermilk, vanilla, and eggs in a blender; process until blended. Add flour mixture; process until thoroughly combined.
Melt brown sugar and butter in a 12-inch cast-iron or ovenproof skillet coated with cooking spray over medium-high heat. Add apple and cinnamon; cook 1 to 2 minutes or until apple begins to soften.
Pour egg batter evenly over apple in pan; sprinkle with pecans. Bake at 400° for 15 minutes or until top of cake is lightly browned. Remove from oven; sprinkle with powdered sugar.