Weigh the flour and sugar in a bowl. Add the butter and rub together with your fingertips until the mixture resembles wet sand, with no buttery lumps. Beat the egg with the vanilla, then add to the bowl. Mix briefly with a cutlery knife to combine, then use your hands to knead the dough together – try not to overwork the dough, or the biscuits will be tough. Shape into a disc, then wrap in cling film and chill for at least 15 mins. Heat oven to 180C/160C fan/gas 4. Line two baking sheets with baking parchment.
Dust a work surface with flour. Halve the dough, then roll one half out to the thickness of a £1 coin. Use an egg-shaped cookie cutter (ours was 10cm long; you could also make a cardboard template to cut around) to stamp out as many cookies as you can, then transfer them to one of the baking sheets, leaving a little space between the biscuits. Repeat with the other half of the dough. If you want to make jammy biscuits, use a small circular cutter to stamp holes in half of the biscuits (where the yolk would be). If you intend to make both iced and jammy biscuits, only stamp holes in a quarter of the biscuits.
Bake for 12-15 mins, until the biscuits are pale gold. Cool on the sheets for 10 mins, then transfer to a wire rack to cool fully. Once cool, decorate to your liking (see next steps). Will keep in an airtight container for up to five days.
To decorate the biscuits with icing, add enough water to the icing sugar to make a thick icing – it should hold its shape without spreading when piped. Transfer about a third of the icing to a piping bag fitted with a very small round nozzle (or just snip a tiny opening at the tip). Pipe an outline around the biscuits, then draw patterns in the middle – lines, spots and zigzags work well. Leave to dry for 10 mins. Divide the remaining icing between as many colours as you’d like to use, then use the gels to dye them. Loosen each icing with a few drops of water, then transfer them to piping bags. Use the coloured icing to fill the empty spaces on the biscuits. You may need to use a cocktail stick to tease it into the corners. Once covered, leave to dry for a few hours.
To make the jammy middle biscuits, dust the biscuits with holes in the middle with a heavy coating of icing sugar. Spread the jam or curd generously over the whole biscuits, then sandwich the dusted biscuits on top of them.