Preheat oven to 450 degrees F (230 degrees C). In a large cast iron skillet, heat butter and apples together in oven while preheating. Cook, stirring occasionally, until apples begin to soften.
While apples are cooking, place eggs, flour and milk in a blender or food processor and puree until smooth. Pour batter over apples in preheated skillet and stir briefly to blend with apples. Return skillet to oven and cook for 15 minutes. Reduce heat to 375 degrees and bake for 10 more minutes, until golden brown.