Using a stand mixer or elective whisk, cream the margarine and sugar until light and paler in colour. Whisk in the vanilla.
In a separate bowl mix the flour, cornflour and baking powder, then add to the creamed mixture. Continue to whisk until it forms a wet soft dough. Tip out onto the work surface and mould into a disk. Wrap in clingfilm and cool in the fridge for 1 hour.
Next make the jam. Put a small plate in the freezer. Add the raspberries, caster sugar and 1.5 teaspoons lime juice to a saucepan and bring to a simmer over a low heat. Mash the berries with the back of a spoon. When the sugar has dissolved, increase the heat and bring the mixture to a rolling boil (the bubbles should not settle when stirred). Continue to boil for 6 minutes.
Remove from the heat. To check the jam is ready perform the ‘wrinkle test’. Remove the plate from the freezer and add a teaspoon of jam. Return the plate to the freezer for one minute. The jam should retain it’s shape when pushed. If it doesn’t wrinkle, heat for a further minute.
Add the jam to a sieve to remove the seeds. Then place the bowl in the fridge for a minimum of 20 minutes to cool and set.
Remove the pastry mix from the fridge and generously flour the work surface. Half the dough and roll out to roughly a pound coin / 3mm thickness.
Use a 7.5cm fluted cookie cutter to cut out 14 rounds, roll the remaining dough again and stamp out a further 14 rounds. Use a 2.5cm heart pastry cutter to stamp hearts into the centre of 14 biscuit. Flour the work surface as needed to prevent the pastry from sticking.
Place the biscuits back in the fridge for another 20 minutes. Pre heat the oven to 180C/ 160c fan. Line 2 baking trays with parchment paper.
Bake the biscuits for 14-16 minutes or until they are pale golden in colour. Leave on the baking sheet for 5 minutes, then transfer to a wire rack to cool.
Sprinkle icing sugar over the tops of each biscuit. Then spoon a heaped teaspoon of jam onto the base of the biscuits and spread evenly, leaving a small border around the edges. Firmly sandwich over the top biscuit and serve.