Preheat the oven to 190°C and line two Swiss roll tins with greaseproof paper.
Boil strawberries and jam sugar in a saucepan for 10 minutes then cool in a bowl.
Beat sponge ingredients until fluffy and spread into the prepared tin.
Bake for 18 to 20 minutes until golden-brown and cooked through.
Turn the cake onto a wire rack and peel off the paper to cool.
Beat icing sugar, butter, and milk until smooth and place in a piping bag.
Cut 24 circles of 5cm from the sponge and slice each in half.
Spread jam and pipe buttercream onto twelve halves before topping with remaining circles.
Dust the finished sandwiches with icing sugar.