Preheat the oven to 190°C and line a 23x33cm Swiss roll tin with baking parchment.
Whisk eggs, sugar, and vanilla seeds in a bowl until light, fluffy, and thickened.
Sift the flour over the egg mixture and gently fold it in.
Pour the batter into the lined tin and smooth it out evenly with a spatula.
Bake the sponge for 10 to 12 minutes until just firm to the touch.
Dust a large sheet of baking parchment with caster sugar.
Flip the sponge onto the sugared paper, remove the bottom parchment, and let it cool slightly.
Whip the double cream and vanilla extract until soft peaks form.
Simmer blackberries, sugar, and 60ml of water in a saucepan for five minutes and let cool.
Spread the blackberry compote over the sponge, leaving a 2cm border.
Spread the whipped cream over the compote and sprinkle with whole blackberries.
Roll up the sponge tightly from the long edge, dust with icing sugar, and slice.