Preheat the oven to 170°C (325°F/Gas 3) and line a baking tray with baking paper. Using electric beaters, cream the butter and sugar together until pale and thick. Beat in the egg, then add the vanilla and lemon rind. Add the flour and mix in with a wooden spoon. You can keep the dough in the fridge or make the biscuits immediately.
Break off balls of dough the size of a rum ball, just under 20 g (¾ oz) each. I like them best at 18 g, to be exact. Roll them into smooth balls with your hands. Flatten them ever so slightly, then place on the lined tray, leaving a little space between each. Sprinkle a good pinch of sugar over the tops. Use a plastic bottle cap (from a bottle of water, for example) to gently press a ring pattern on top of each biscuit. This gives them a nice shape when baked. Bake for 10 minutes, until just set but still very pale (they will harden as they cool). Move them to a wire rack to cool.
Mix a little gum arabic with enough water to give a sticky paste. Using a toothpick, put a few drops on the underside of a violet and press gently onto a biscuit.