Remove the stalks and any leaves from small flowers like violas or pansies, and carefully pull apart the roses to separate the petals. Place a sheet of baking parchment on your work surface. Sprinkle the caster sugar over a saucer, then lightly whisk the egg white in a small bowl. Holding a flower or petal with tweezers, use a small paintbrush to paint both sides with egg white. Spoon the sugar over, then shake off the excess and place on the parchment. Repeat with the remaining petals or flowers. Leave to dry for 3 hrs, or overnight if you can. Will keep in an airtight container for up to 1 month, but their colour may start to fade after a few weeks.
Tip the flour into a large bowl and add the butter. Use your fingertips to rub the butter into the flour until you have a damp, crumby texture. Pour in the sugar and gently mix in with your fingertips. Whisk the vanilla, rosewater and egg yolks together in a small bowl with a fork, then drizzle the mixture over the buttery flour. Mix again, squashing the crumbs together to form a dough. If the mixture is a little crumbly, drizzle over 1-2 tsp cold water, then mix again. Tip onto your work surface and knead very briefly until the dough looks even, with no streaks of egg. Wrap in cling film, pat into a round disc and chill for 30 mins.
Heat oven to 180C/160C fan/gas 4 and line two large baking trays with baking parchment. Remove the dough from the fridge – if it’s very firm, leave it at room temperature for 15 mins to soften. Lightly dust the work surface with flour, unwrap the dough and roll it to the thickness of a £1 coin. Stamp out 8cm disks with the fluted side of a biscuit cutter and transfer them to the baking trays. Bake for 10-12 mins, swapping the trays over halfway through cooking. Once lightly golden and firm, remove the biscuits from the oven. Leave to cool on the trays for 10 mins or until stable enough to transfer to a wire rack to cool completely.
Mix the icing sugar with enough water to make a thick icing. Divide the icing between as many bowls as the number of colours you’d like to use, and dye each one a pastel shade with a drop of food colouring. Transfer half of one of the icings to a disposable piping bag fitted with a fine writing nozzle. Pipe a ring around the outside of the biscuit and set aside to dry. Continue until you’ve iced a third of the biscuits (if you’re using three colours), then move on to the next colour, transferring any leftover icing back to its original bowl. Leave the biscuits to dry for 10-15 mins.
Add 2-3 tsp water to each icing to make it a little runnier. Spoon the icing onto the biscuits, matching the colour to the rings. Encourage the icing to flood the surface by easing it to the edges with your spoon – don’t be too generous with the icing as it may overspill when you add the flowers and petals. You can now dust the surface of each biscuit with a little shimmer dust, if you like, then top with the flowers. Leave the biscuits to set for at least 1 hr before wrapping in tissue paper in a pretty box. Will keep for up to 1 week.