Sift the flour, salt and baking powder into a bowl (or a food processor bowl). Dip the butter into the flour and then grate it finely into the flour, dip and grate until all the butter as been added. Stir the butter in (or pulse it a couple of times) and break up any larger lumps. Stir or pulse in all the buttermilk (except 1 tbsp) to make a rough, crumbly mixture. Flour the work surface and tip the mixture onto it in a pile, pat it into a rectangle and roll over it with a rolling pin once or twice until it is about 2cm thick. Cut the mixture into thirds. Scatter half of the sweetcorn mix on one third, lift another third on top, scatter on the rest of the sweetcorn and put the final piece of dough on top. Roll again briefly (the less you roll the better) and then cut the mixture into 8 squares, rectangles or triangles.