Rub the butter into the flour with a pinch of salt, then stir in the sugar and currants.Tip in the egg and mix into a dough. Grease a flat cast iron griddle pan or large heavy-based frying pan (not non stick) with a drizzle of oil and put on a very low heat.
On a floured surface, roll out the dough to a 1cm thickness. Stamp out thin rounds with a 6.5in fluted cutter, re-rolling the trimmings as you go.
To check whether the heats right for the griddle or pan you need to cook a trial cake. The pan is hot enough when a sprinkling of water sizzles up in a few seconds. Drop a welsh cake on to the griddle, leave it there for 5-6 minutes till the underside is brown, then flip it over and do the same to the other side to cook the middle. If the cakes are browning too quickly, reduce the heat of the pan & allow to cool slightly before you cook any more.
Cook the Welsh cakes in batches, trying not to overcrowd the griddle, then transfer to a wire rack to cool. Serve warm sprinkled with caster sugar.