Method

1. Preheat oven to 160°C/315°F (140°C/285°F fan-forced). Line two large oven trays with non-stick baking paper.
   
2. Use an electric mixer with a paddle attachment to beat the butter, sugar and vanilla until pale and creamy. Add the egg yolk and beat well. Sift together the flour and salt. Add to the butter mixture and beat on lowest possible speed until just combined and a soft dough forms.
   
3. Divide the dough in half. Use a rolling pin to roll out one portion of dough between two sheets of non-stick baking paper to 5mm/¼in thick. Use a 6cm/2¼in fluted round cutter to cut out rounds of dough. Use a palette knife to transfer the rounds to the lined trays, leaving about 2cm/¾i between each to allow for spreading. Repeat with the remaining dough portion, re-rolling off-cuts, to make about 30 rounds in total. Place the trays in the fridge for 30 minutes to chill.
   
4. Use a 2cm/¾in heart, fluted round or star-shaped cutter to cut the centre from half of the chilled cookie rounds.
   
5. Bake for 25-30 minutes, swapping the trays halfway through baking, or until pale golden and cooked through. Cool on the trays.
   
6. Spoon 1 teaspoon of the warm jam onto each of the cookie bases and spread to about 1cm/½in from the edge. Dust the cookie tops (with the cut-outs) generously with icing sugar. Carefully sandwich the cookies together, pressing gently to join.