Whisk the egg whites in a large bowl until they form soft peaks. Put the yolks, vanilla extract and 1 tablespoon of the sugar in a separate bowl and use the same whisk to beat them together. Fold a spoonful of the egg whites into the yolks to loosen the mixture, then add the remainder and fold in gently with a large metal spoon.
Melt half the butter in a 20 cm (8 inch) frying pan. As soon as it stops foaming, swirl in half the egg mixture and cook over a medium heat for 3–4 minutes until the underside is golden, then transfer to a preheated hot grill to brown the top lightly. Carefully slide the omelette on to a warmed serving plate and keep warm in a moderate oven. Repeat with the remaining ingredients to make a second omelette.
Dot the omelettes with the jam and berries, then fold in half to enclose the filling. Dust the tops with the remaining sugar, cut in half and serve immediately with cream.