Beat together the butter and sugar until pale and creamy, then beat in the yolk, vanilla and zest. Sift the flour and salt into the bowl, then stir in to make a soft dough. Split into two flat discs, then wrap in cling film and chill for 30 mins, or until firm but not rock hard. Meanwhile, heat oven to 180C/160C fan/gas 4. Line two baking sheets with baking parchment.
Flour the work surface, then roll the dough to the thickness of 2 x £2 coins. Stamp circles with an 8cm cutter and re-roll the trimmings. If you like, pinch a speech bubble point into the bottom of each round. Chill for 10 mins, or until firm, then poke cake pop sticks carefully into the dough. (I find putting one hand on top of the dough as I insert the stick with the other prevents the stick from popping through the surface of the dough.)
Bake for 12 mins, until pale golden. Leave on the tray for 5 mins before lifting to a wire rack (use a palette knife rather than the sticks) and cooling completely. Make the icing by beating sugar and egg white until thick and smooth. Remove half to another bowl, colour it, then spoon into a disposable piping bag and snip off the tip (or use a number 2 nozzle). Pipe a speech bubble border around each cookie and leave to set for a few mins.
Now loosen the white icing with a few drops of water until runny. Spoon a little onto each cookie and let it flood to the outline, nudging it up to the edge if needed using a cocktail stick or tip of a teaspoon. Dry for 10 mins, then pipe captions on top. Leave to dry. Keep the cookies in an airtight tin for up to 3 days.