For the jellyroll: Line a baking sheet (30X35 cm) with parchment paper. Preheat oven 180°C/ 350°F).
Separate eggs by directly adding the egg whites into a large mixing bowl. Add the egg yolks into a cup. Sift flour and baking powder.
Add the salt and lemon juice to the egg whites and beat at high speed with your electrical mixer until it reaches the perfect foamy stiffness. While still beating gradually add the sugar, vanilla sugar and egg yolks.
Reduce speed to low. Add the flour mixture and beat for one minute only to blend all ingredients.
Evenly spread onto the baking sheet and bake 10-12 minutes. While the cake is baking, spread a clean dry kitchen towel on your working surface. Dust with confectioner’s sugar. This will prevent the cake from sticking on the towel.
Carefully remove the cake from the baking sheet with parchment still on it, holding it with your hands and turning it onto the towel. Carefully remove parchment paper and dust the cake with confectioner’s sugar. Roll immediately and let cool for about 30 minutes.
Prepare the filling: In a large mixing bowl whip heavy cream with firming agent (if using, see Forêt Noire) until very stiff, and mix in the strawberries. Carefully unfold the jellyroll but don’t open completely to avoid breaking it. Spread with the filling and roll again. Thinly cut on both sides. Transfer to a serving platter and chill for 2 hours. Dust with confectioner’s sugar before serving. If you wish to cover the roll with whipped cream, add 100g heavy cream to the ingredient. Reserve about 4 tablespoons whipped cream on the side before mixing in the strawberries.