Separate eggs, keeping eggwhites to one side. Place butter, sugar and lemon zest into a food processor, then blitz to make a creamy paste. Add egg yolks and flour and process mix to a soft dough, adding a splash of milk or water if the mixture is too stiff. Place dough in a bowl, cover with cling film and refrigerate for 30 minutes to allow dough to firm up. Preheat oven to 180°C. Place dough on a lightly floured surface and roll out until dough is 1cm thick, then cut out desired biscuit shapes - such as hearts - re-rolling pastry to make more biscuits. Lay biscuits on a lightly greased baking tray, brush biscuits with lightly beaten eggwhite and sprinkle with coconut. Bake for 15-20 minutes or until golden and crisp.