1. Sift the flour and sugar into a bowl. Add the butter and work together until the mixture binds into a dough.
2. Form into walnut-sized balls.
3. To make the traditional shape that gives these cakes their name, use a floured and sugared shell similar to a flat scallop shell.
4. Press the dough balls in the shell, then remove them carefully and place on a floured baking sheet.
5. Bake in a preheated moderate oven 180°C (350°F) Gas Mark 4 for 20 minutes or until the cakes are just beginning to colour.
6. Cool slightly, then toss in caster sugar.
Note: The tradition is that when Prince Llyw-elyn held Court in Aberffraw, his Queen went walking on the beach with her attendants and found a shell. She asked to have cakes baked in the same shape and they were called Teisen Aberffraw.