Chocolate & raspberry sandwich biscuits

1. For the filling, gently heat the raspberries, sugar, lemon zest and juice and 2 tbsp water in a pan. Stir until it begins to boil. Reduce the heat and simmer for 10-15 minutes, stirring regularly until thickened. Pass through a sieve to remove the seeds.

2. Put the cornflour in a small bowl and mix in 2 tbsp water to make
a paste. Stir into the sieved raspberry mixture and pour into a clean saucepan. Put over a medium-low heat and stir constantly until the mixture begins to boil, then reduce the heat to low. Continue stirring and cook for 2 minutes more. Pour into a container and leave to cool, then cover and chill for at least 1 hour, or preferably overnight.

3. Preheat the oven to 170ºC, gas mark 3. Line 2 large baking trays with baking parchment. Put the butter, icing sugar, vanilla and salt in a mixing bowl and beat together until light and fluffy. Stir in the flour and cocoa powder to make a dough. Wrap and chill for at least 1 hour, or until firm.

4. Divide the dough into 34 pieces (each roughly 20g) and, using damp hands, roll into balls. Arrange on the baking trays, at least 3cm apart (you may need to do 2 batches). Using the back of a spoon, slightly flatten them so they are about 4cm wide. Bake for 20 minutes, until firm. Leave to cool completely on the trays.

5. Use a piping bag with a 1cm nozzle to pipe the filling on the flat side of half the biscuits, then sandwich with the other biscuits. If not eating at once, keep the unfilled biscuits in an airtight tin and fill just before serving.