1. Butter a 2-quart (2l) gratin or baking with the softened butter. Sprinkle the granulated sugar in the baking dish and tilt the dish, so the sugar coats the bottoms and sides of the dish. Spread the jam in an even layer over the bottom of the dish and place the dish on a baking sheet. Set aside.
2. Preheat the oven to 375ºF (190ºC). Set the oven rack near the top third of the oven.
3. In the bowl of a stand mixer, or by hand in a large bowl and a whisk, beat the egg whites on medium speed with the salt until they’re foamy. Increase the speed to high and continue beating until the egg whites start to hold their shape. While the mixer is running, or while whisking, gradually whisk in the superfine sugar, continuing to whip until the meringue is smooth and glossy. Near the end of the whipping, beat in the vanilla sugar or vanilla extract or paste.
4. In a small bowl, beat the egg yolks with a fork until liquidy. Using a whisk, gently, but thoroughly, incorporate the egg yolks into the egg whites, making sure to reach down to the bottom of the mixing bowl so everything is incorporated. Using a flexible silicone spatula, sprinkle the flour over the meringue while gradually folding it in, making sure (again) to reach down to the bottom of the bowl so everything is completely combined.
5. Using a plastic bench scraper (or a flexible spatula) lift out one-third of the egg mixture forming it into an oval shape using the rounded curve of the inside of the bowl, to help you form the oval. Place the oval on one end of the jam-covered baking dish. Make another oval mound with half of the remaining meringue and place it on the other end of the baking dish. Finally take the remaining meringue and place it in the middle of the baking dish. You should have three large, oval mounds. (Although don’t worry, it doesn’t need to look perfect.)
6. Bake for 11 to 14 minutes, or until the souffléd meringue is browned and cooked to your liking. You can stick a paring knife between two of the mounds to take a look and see. (I found about 13 minutes was just right for me, but if you like meringue more cooked, lean toward 14 or even 15 minutes.) Remove from the oven, sprinkle with powdered sugar, and serve immediately.