Place the flour and mixed spice (and baking powder if using plain flour) in a bowl and stir to combine.
Add the cold cubed butter and rub in using your fingertips until the mixture resembles fine breadcrumbs and no lumps of fat remain.
Add the sugar and sultanas or currants and stir through.
Gradually stir in enough milk to bring the mixture together into a ball of dough. Be careful not to add too much, it should be soft but not sticky.
Roll out the dough on a lightly floured surface until it is about 5mm/¼in thick. Cut it into rounds using a 6.5cm/2 ½in to 7.5cm/3in fluted cutter.
Bring the scraps together gently and roll out again to cut out more rounds.
Place a cast iron griddle or heavy bottomed frying pan over a low-medium heat and brush with a small amount of butter. Place some caster sugar in a bowl for dusting.
Cook a few Welsh cakes at a time until golden, about 3 minutes. Then carefully flip them over and cook for about 3 minutes more until golden on both sides and cooked through. They firm up as they cool. Keep a close eye on them, they can go from golden to burnt very quickly!
It can be tricky to get the pan to the right temperature so it is a good idea to cook a test cake first to make sure it's not too hot.
Once the Welsh cakes are cooked, roll them in the caster sugar while still hot and repeat with the rest of the cakes.