Jam Sandwich Biscuits

Jam

• 75 g fresh strawberries, hulled, halved
• 75 g fresh raspberries
• 100 g granulated sugar
• ½ Tbsp lemon juice

Biscuits

• 150 g unsalted butter, diced
• 100 g caster sugar, plus 1 Tbsp
• 250 g plain flour, plus extra for dusting
• 2 egg yolks, from medium eggs
• 1 tsp natural vanilla extract
• ½ tsp fine sea salt