Place the first seven ingredients into the bowl of a food processor, and whirl briefly. Cut the butter into four chunks, and pulse with the dry mixture until it forms large clumps. Whisk together the buttermilk and egg yolk, and add to the processor bowl; process only until mixture barely clings together.
Transfer the dough to a lightly floured work surface, and gently and quickly knead in the raisins. Form the dough into a flat disk about 7 inches across, and cut it into small or large wedges, depending on your appetite or waist size. Transfer the disk to a parchment-lined or lightly greased baking sheet, and separate the wedges by pulling them apart about 1 inch, but still keeping them in a circle.
Brush the tops of the scones with beaten egg white, and sprinkle with coarse sugar. Bake them in a preheated 375°F oven for 25 to 27 minutes (for 8 scones; a little less for smaller scones), inspecting at midpoint to admire and turn. Remove the scones from the oven when they're a light, golden brown, and cool them on a wire rack.
Note: You don't need a food processor to make these scones. Simply mix the dry ingredients with the butter till crumbly, using a mixer, a pastry blender, or your fingers. Stir in the buttermilk and egg yolk, knead in the raisins, and proceed as directed above.