Heat oven to 200C/180C fan/gas 6. Put the butter in a bowl and beat it using an electric whisk until soft and creamy. Beat in the sugar, then the egg and vanilla or lemon, and finally the flour to make a dough. If the dough feels a bit sticky, add a little more flour and knead it in.
Cut the dough into six pieces and roll out one at a time to about 5mm thickness on a floured surface. The easiest way to do this is to roll the mixture out on a baking mat. Cut out letter and number shapes (we used 7 x 4cm cutters) and peel away the leftover dough at the edges. Re-roll any off-cuts and repeat.
Transfer the whole mat or the individual biscuits to two baking sheets (transfer them to baking parchment if not using a mat) and bake for 7-10 mins or until the edges are just brown. Leave to cool completely and repeat with the rest of the dough. You should be able to fit about 12 on each sheet. If you are using two sheets, then the one underneath will take a minute longer.
Ice the biscuits using the pens to make stripes or dots, or colour in the whole biscuit if you like. They will keep for five days in an airtight container.