Step 1

Preheat the oven to 425°F and set an oven rack in the middle position.

Step 2

Prepare the skillet: Spread the softened butter over the inside of a 10-inch ovenproof skillet. Sprinkle with the sugar, shaking to evenly coat the pan.

Step 3

In a small microwave-safe bowl, combine the raisins and rum. Microwave for 30 seconds, then set aside.

Step 4

In the bowl of an electric mixer fitted with the whisk attachment or beaters, combine the egg whites, salt, and sugar. Beat on medium-high speed until soft peaks form, a few minutes (peaks should droop softly when lifting the whisk).

Step 5

Meanwhile, in a large bowl, whisk the egg yolks, milk, vanilla, and 1 tablespoon of the rum from the rum-steeped raisins. Add the flour and butter and whisk until evenly combined.

Step 6

Lighten the yolk mixture by whisking in one-quarter of the beaten egg whites. Using a rubber spatula, gently fold in the remaining egg whites until evenly combined.

Step 7

Pour the batter into the prepared skillet. Cook over medium heat for 2 minutes. Evenly distribute the rum-steeped raisins on top (discard the remaining rum in the bowl), then transfer the skillet to the oven and bake for 14 to 16 minutes, until puffed and lightly golden. Remove from the oven and immediately place an oven mitt or dishtowel over the handle of the skillet to remind yourself that it's hot (it's easy to forget and burn yourself).

Step 8

For immediate serving: Spoon the Kaiserschmarrn into bowls, dust with confectioners' sugar, and serve with fruit, if desired.

Step 9

For caramelized Kaiserschmarrn (recommended): Melt the butter in a large skillet over medium heat. Using a spatula, cut the baked Kaiserschmarrn into 2-inch pieces in its skillet, then transfer to the hot skillet. Sprinkle with the granulated sugar and cook, stirring occasionally, until the pieces are golden and caramelized. Dust with confectioners' sugar and serve with fruit, if desired.

Step 10

Make-Ahead Instructions: You can prepare the kaiserschmarnn base up to one hour ahead of serving. After baking, allow it to cool slightly, then cover it loosely with a clean kitchen towel or aluminum foil and keep it at room temperature. When you're ready to serve, cut the pancake into 2-inch pieces right in the skillet and proceed with the caramelization instructions.