Heat oven to 220C/200C fan/gas 7. Sieve the flour, ginger and a pinch of salt into a large mixing bowl. Add the butter and cut into the mixture using a round-ended knife. Then use your fingers to rub the butter into the flour until it resembles fine breadcrumbs. Add the caster sugar and mix to combine.
Make a well in the middle, add the buttermilk and stir in using the knife again. Once it comes together, use your hands to bring the dough into a ball. Tip out onto the work surface and very lightly knead for 30 secs until almost smooth.
Lightly dust the work surface with flour and roll or flatten the dough into a square roughly 2cm thick. Using a long knife, trim the edges, then cut the dough into 6-7cm squares. Arrange the squares on a baking sheet, brush the tops with the beaten egg white, sprinkle with granulated sugar and bake on the middle shelf of the oven for about 10 mins until well risen and golden brown. Serve warm with Rhubarb & ginger jam and clotted cream.