Roughly chop 1kg strawberries, place in a heatproof bowl with 1/3 cup (75g) caster sugar and 1 tsp salt, and toss to combine.
Cover the bowl with 2 layers of plastic wrap. Place the bowl over a saucepan of simmering water (don’t let the bowl touch the water) and cook over low heat for 1 hour or until the strawberry releases its juice.
Strain the strawberry liquid through a fine sieve lined with muslin. Fold the muslin over the pulp, place the sieve over a bowl and weigh down with a can. Set aside for 2 hours for all the liquid to drain into the bowl. Transfer the strawberry liquid and pulp to separate containers. Refrigerate the liquid and freeze pulp overnight.
Combine vanilla seeds with brown sugar and 1/4 tsp salt in a bowl. Rub together with your fingers until well combined, then set vanilla sugar aside.
The next day, finely chop remaining 250g strawberries and set aside.
Place frozen strawberry pulp in a food processor with the remaining 2 tbs caster sugar and whiz on high speed to form a frozen dust or crumble. With the motor running, add eggwhites and whiz until pale pink and a thick sorbet consistency. This needs to be served immediately; otherwise, freeze until ready to use.
Divide spoonfuls of sour cream among 4 bowls. Sprinkle with vanilla sugar and scatter over the finely chopped strawberry. Place a scoop of strawberry sorbet on top of the sour cream, then pour the strawberry soup around the outside of the sour cream and sorbet. Garnish with basil leaves to serve.
