1 Separate the eggs. Combine the egg whites and salt in a bowl or a tall vessel. Beat with a hand mixer until stiff peaks form, then gradually add 100 grams (approximately 1/2 cup) sugar. Stir in the egg yolks briefly.
2 Mix together the flour and cornstarch in a bowl. Sift through a sieve into the beaten egg whites, then combine with a whisk.
3 Line a baking sheet with parchment paper and spread the batter on top evenly with a spatula. Bake in a preheated oven at 200°C (fan: 180°C, gas mark 3)(approximately 400°F/convection 350°F), on the middle rack, 8-10 minutes.
4 Sprinkle a kitchen towel with the remaining sugar and invert cake onto towel. Peel off the parchment paper and roll up the sponge cake, incorporating the towel while rolling. Let cool completely on a wire rack.
5 Soak the gelatin in a bowl with plenty of cold water until softened.
6 Meanwhile, rinse the lemon in hot water, wipe dry and finely grate the zest.
7 Mix together the buttermilk, yogurt and lemon zest in another bowl. Add sweetener to taste and beat until fluffy with the hand mixer.
8 Measure out 2 tablespoons of the buttermilk mixture and place in a pot. Heat until warm, then add the softened gelatin and stir with a wooden spoon until dissolved. Add another 2 tablespoons of the buttermilk mixture and stir to combine, then stir in the rest of the mixture, remove from heat and let cool slightly.
9 When the cream begins to set, unroll the sponge cake carefully and spread the buttermilk cream on top with the spatula.
10 Rinse blackberries and drain well in a sieve. Scatter 200 grams (approximately 7 ounces) of blackberries over the cream and roll up the sponge firmly. Refrigerate at least 2 hours.
11 Dust the roulade with powdered sugar using a tea strainer, garnish with remaining blackberries, slice and serve.