Preheat the oven to 170°C and line two baking trays with parchment.
Cream the butter, sugar, and vanilla extract until pale and fluffy.
Fold in the flour and cornflour, knead gently until smooth, and chill for 30 minutes.
Roll dough to 5mm thick, cut into 48 circles, and bake for 15–20 minutes until golden.
Sprinkle with caster sugar, cool for 10 minutes, then move to a wire rack.
Beat butter, milk, vanilla, and icing sugar together until smooth to make buttercream.
Boil mashed strawberries, sugar, and lemon juice for 5 minutes, then cool.
Sandwich two shortbread rounds with buttercream and jam, then top with sliced strawberries.