Preheat the oven to 180°C/160°C Fan/Gas 4. Line two 20cm cake tins with greaseproof paper. Mix the spread with the sweetener until smooth and then gradually beat in the eggs and vanilla essence. Sift the flour and fold into the mixture.
Divide the mixture between the cake tins and bake for 25-30 minutes (until the sponge springs back when lightly touched). Remove from the oven and turn out onto a wire rack to cool completely.
Mix the quark with the yogurt; halve most of the strawberries and slice the remainder. Place one sponge on a plate, top with the strawberry halves and pour over the yogurt mixture. Place the second sponge on top and decorate with the sliced strawberries. Dust with a little icing sugar and serve immediately.