500g/1lb 2oz floury potatoes (such as Maris Piper), unpeeled, cut into large chunks
150–175²µ/5½–6΄Η³ϊ plain flour, plus extra for dusting
1 tsp baking powder
sea salt and freshly ground black pepper
50²µ/1ΒΎ΄Η³ϊ butter, plus extra for frying and to serve
1 free-range egg, beaten