Dissolve sugar in boiling water in a small pan, boil briefly, then cool completely.
Mix cooled syrup with yoghurt and lemon juice, then churn in an ice cream machine for 15 minutes.
Preheat the oven to 200°C.
Pulse sugar, flour, butter, and nuts in a food processor until the mixture looks like coarse breadcrumbs.
Press half the crumb mixture into the bottom of a 20cm round non-stick tin.
Beat the remaining crumbs with egg, baking powder, and soured cream to form a smooth batter.
Pour batter over the base, sprinkle with extra sugar, and bake for 25 minutes.
Cool to room temperature, slice into wedges, and serve with chopped nuts and yoghurt sorbet.