For the strawberry jam

1. Put the hulled and quartered strawberries in a large saucepan. Add the sugar, water, and lemon juice and mix to combine.
2. Bring to a boil over high heat and let boil for 5 minutes. Reduce to medium-low heat and simmer for approximately 1 hour, until the mixture reaches a temperature of 105°C / 220°F.
3. While still hot, pour into warm, sterilised jars. Tighten the lids and leave to cool overnight.

To make the shortbread

1. Beat the butter and icing sugar together using the paddle attachment of your stand mixer or with an electric hand mixer, until fluffy and pale, about 3 minutes. Add the vanilla and egg yolk and beat to combine.
2. Sift in the flours and mix by hand with a wooden spoon until combined. Chill the dough in the fridge for 1 hour.
3. Preheat the oven to 190°C / 370°F and line cookie sheets with parchment paper.
4. Lightly dust your worktop with icing sugar and roll out the cookie dough to 3 mm thick. Use a fluted 5 cm cookie cutter to cut out circles. Use a small round cutter (or heart-shaped if you have one) to cut out the centre of half the cookies — the wide end of a piping nozzle works well for this. It’s up to you how big to make the hole.
5. Bake for 12–15 minutes, until pale golden. Transfer to a wire rack to cool, and sprinkle with the white sugar while still warm.
6. When cooled, spread 1/2 tbsp of jam on the cookies without a hole, and sandwich with one of the cut-out cookies. Repeat with all cookies.