Combine raisins and rum in a small bowl and set aside to soak.
Whisk flour and milk in a large bowl until well combined. Set aside for flour to hydrate for 10 minutes.
Whisk egg yolks and salt into the moistened flour.
Preheat the oven to 350 degrees F (175 degrees C).
Beat egg whites in a glass, metal, or ceramic bowl until foamy. Gradually add sugar, continuing to beat until stiff peaks form. Fold egg whites into the batter in 2 batches.
Heat an oven-safe, heavy skillet over medium heat. Melt butter and pour in the batter. Sprinkle rum-soaked raisins on top. Let batter gently brown, 4 to 5 minutes. Reduce heat if batter browns too fast. Cut into quarters with a large spatula. Flip the quarters and let the other side brown gently, 4 to 5 minutes.
Transfer the skillet into the oven and bake until pancake is set and puffed up, about 8 minutes. Use 2 large forks to tear the pancake into bite-sized pieces. Sprinkle with confectioners' sugar and serve with applesauce.