Whisk the egg yolks with milk, vanilla, sugar and salt, then whisk in the flour until just smooth. Rest the batter for 15 minutes.
Beat the egg whites with a pinch of salt to soft peaks. Fold into the rested batter in three additions — stop while a few streaks remain.
Cook
Melt half the butter in a 26–28 cm oven-safe pan over medium heat. Pour in the batter, scatter over the drained rum raisins, cover and cook 6–8 minutes until the underside is golden and the top is just set at the edges.
Quarter the pancake in the pan, flip each quarter, add the remaining butter and cook 3–4 minutes more until golden.
Caramelise
Tear the pancake into rough bite-sized pieces with two forks. Sprinkle with the sugar and either toss over high heat for 2 minutes or slide under a hot grill for 60–90 seconds — the pieces are done when the edges are crisp and caramel-brown.
Serve
Dust generously with powdered sugar and serve straight from the pan with plum compote or apple sauce.
Notes
The batter can be made (without the egg whites) up to 2 hours ahead. Whip and fold in the whites just before cooking. Leftovers reheat surprisingly well in a buttered pan, though in my experience there are never any.