A couple hours in advance (or a day prior) combine the raisins and the rum in a small container to allow them to plump. As soon as the raisins are plumped, drain the excess rum and reserve the raisins.
Set up your oven so that there is one rack in the center with no racks above it. Place the All-Clad Oval Roaster on the rack and preheat the oven to 375 degrees.
In a mixing bowl, whisk together the egg yolks, vanilla, milk, lemon zest and salt.
In a separate bowl, whip the egg whites to medium stiff peaks. Gently whisk the flour into the yolk and milk mixture just until it has been incorporated, do not over mix. Use a rubber spatula to fold the egg whites into the batter one third at a time.
As soon as the batter has been mixed, remove the Oval Roaster from the oven. Spray the pan and its sides with vegetable oil spray then pour the batter into the pan. Sprinkle the raisins over the top of the batter and return the pan to the oven. Bake for about 15 minutes or until the top of the pancake is almost completely cooked.
Remove the pan from the oven and sprinkle the top with half of the granulated sugar. Use a large spatula to flip the pancake over. It helps splitting the pancake in half or even quarters to do this. If the pancake breaks it is completely OK.
Sprinkle the other side with the remaining granulated sugar and distribute the cubes of butter evenly around the pan. Return the pan to the oven for another 10 minutes.
Remove the pan from the oven and place over a medium-high burner. Working quickly, use two forks to tear the pancake into pieces about 1 to 1-1/2 inches in size. Stir the pancake around while over the heat until it begins to lightly brown and caramelize. Turn off the heat and mound the pieces of the pancake into rustic pile in the center of the pan. Dust the pancake generously with powdered sugar.
Serve immediately with plum or apricot jam and vanilla ice cream.