Preheat your oven to 180C (160C fan).
Cut the butter into small pieces then add to a large bowl with the flour, salt, sugar and baking powder. Rub together with your fingers until fine breadcrumbs are formed.
Combine the milk/buttermilk, egg and yoghurt together and pour into the breadcrumb mixture.
Add the sultanas to the bowl and using the Hand Mixer with dough hooks, slowly on speed 1, combine until a soft dough forms (do not overwork the dough).
Remove the dough and place on a floured surface. Roll out the dough until roughly 2-3cm thick.
Gently press down and cut the scone shapes with a cutter and then place on a floured baking sheet.
Brush the tops using an egg wash or milk.
Bake in the preheated oven for 15-20 mins.
Serve warm with clotted cream and Tiptree strawberry jam.
