Place blackcurrants in a saucepan with 100g (4 oz) sugar and water. Bring to the boil. Stir a couple of times, then add cornflour slurry. Stir well and then reduce heat and cook for 3 minutes. Pour into round ovenproof dish and place on one side to cool.
Preheat oven to 200 C / Gas 6.
Mix mascarpone with remaining sugar till well blended. Carefully drop 6 spoonfuls of the mascarpone over blackcurrants trying to space evenly. Do not spread together.
Sift flour, baking powder and salt. Add demerara sugar and lemon zest. Stir in lemon juice and melted butter. Then add milk and mix gently. Place in 6 blobs over mascarpone.
Cook for 25 minutes in the preheated oven. Before serving let stand for 5 to 10 minutes.