Combine flour and butter and rub together till the mixture resembles breadcrumbs. Add the icing sugar and mix to combine. Gather mixture into a ball, kneading slightly if needed, then wrap in cling film and chill in the fridge for 1 hour or up to overnight.
Preheat oven to 180 C / Gas 4. Line a baking tray with parchment.
Dust a work surface with flour. Roll out half of the biscuit dough to 3-5mm thick. Cut out 6 rounds using a cutter, tin or glass. Repeat with the remaining dough. Transfer rounds to the prepared baking tray.
Bake biscuits for 10 to 12 minutes, or till just golden around the edges. Remove from oven and let cool completely on the tray.
Lay out 6 cooled biscuits bottom side facing up. Spread each with your favourite jam, then sandwich with another biscuit (bottom side touching the jam).
Make the icing by combining the icing sugar with water or milk. Start with 1 to 2 teaspoons, mix well, then add more liquid only if needed to reach a spreading consistency. If the icing becomes too thin, add more icing sugar.
Spread the top of each biscuit with the icing sugar. Set biscuits aside for the icing to set at room temperature.
For a traditional finish, top each biscuit with a glace cherry half before the icing sets.