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Line a couple of trays with baking parchment and heat oven to 180C/160C fan/gas 4. Mix together the flour, salt, caster sugar and lime zest (keep the zested limes for the icing). Add the butter and rub together until the mixture resembles fine breadcrumbs.
Beat the egg and vanilla bean paste together with a fork, then mix into the other ingredients. Knead until even. Chill until firm, then roll on a lightly floured surface until about 4-5mm thick. Stamp out 8cm rounds, transfer to baking trays, then chill again for 15 mins before baking for 15-17 mins until golden. Remove from the oven, transfer to a wire rack and leave to cool before decorating.
For the icing, put the royal icing sugar into a bowl, add about 65ml lemon or lime juice and use a metal spoon or electric mixer to beat until it is fairly thick and glossy. If the icing is too thick though, it will be hard to pipe. Take a quarter of the icing, put into a piping bag fitted with a No 2 (very small round) piping nozzle and set aside. To see step-by-step images, please see the tips, below left, for a link to our guide page.
For the flood icing, add another 10ml lemon or lime juice to the remaining icing, a few drops at a time until it is pourable. You can now divide this icing into as many bowls as you like and colour by mixing in a little gel food colouring.
Place the coloured icing into three piping bags, ready for decorating.
To decorate the biscuits, use the thicker icing to draw your outline or design onto the biscuits. Let the icing set for a few minutes.
Pipe enough runnier icing to cover the entire biscuit – don’t worry about it being neat at this point. Use a toothpick to spread the icing evenly inside the hard icing ‘wall’. If the surface of the icing isn’t flat, gently shake the biscuit back and forth until you have a smooth covering.
To add a simple dot decoration, while the icing is still wet, use the runnier icing again to pipe dots again – this will sink on, leaving you with a smooth effect. Pop the biscuits onto a baking tray and place into an oven at its lowest setting for about 30 mins to allow the icing to set hard.