Set oven to 160°C (fan). Line two trays with parchment paper.
Beat softened butter and caster sugar until light and fluffy (2–3 mins).
Mix in the egg until smooth.
Stir in sifted flour and salt. Add a bit more flour if dough is sticky.
Wrap in cling film and chill for 20 mins.
Roll out to 0.5cm thickness on a floured surface.
Cut rounds with a 6cm fluted cutter. Place on trays with space between.
Bake for 10–12 mins until pale golden.
Cool on tray for 2 mins, then transfer to a wire rack.
Spread raspberry jam on one biscuit, top with a second.
Mix icing sugar with 2–3 tbsp water until thick but spreadable.
Top each biscuit with icing and half a glacé cherry or jam.