2–3 tablespoons XUXU
125 g icing sugar
1. Heat oven to 180°. Whisk butter, sugar, salt and grated lime zest until light and fluffy.
2. Add egg yolks one at a time and mix. Finally add flour stirring constantly.
3. Place the mixture into a piping bag with a wide nozzle and pipe rings on to a baking tray lined with baking paper. Bake in the oven for 12 to 15 minutes, allow to cool.
4. Mix the icing sugar with XUXU until smooth, carefully spread icing on to one half of each biscuit.