Slit the vanilla bean down the center. Bring the milk with vanilla bean to a boil and then remove from heat. Beat the egg yolks with sugar until foamy and stir in the flour. In a thin stream, pour in the hot milk (discarding the vanilla bean) while stirring. Simmer for a few minutes, until thickened (do not boil), then cool. Zest the lemons and mix with lemon juice and sugar in a pot. Add just enough water until the sugar is covered and simmer over low heat. Beat the egg whites with salt until very stiff. Gradually fold in the vanilla cream and cooled lemon syrup. Grease 1 large or 4 small soufflé dishes, dust lightly with sugar and pour in the batter. Place in the oven and bake at 200°C (approximately 375°F). When the souffle has risen (15-20 minutes), turn the temperature down to 180°C (approximately 350°F) and continue to bake for about 20 minutes (baking the small soufflés for slightly less time).
Remove from the oven, decorate the soufflé border with hazelnuts and top with whipped cream and lemon slices. Serve immediately.