Heat oven to 500 degrees, rack in the middle position. Combine flour, baking powder, and salt. To help achieve that very light texture, sift the flour mixture five times – and set it aside in a large mixing bowl.
In another bowl, whisk together buttermilk, olive oil, sugar and one egg. It’s important to have everything ready to go before you mix the liquid into the flour, as this starts activating the baking powder.
In separate containers, measure raisons and raspberries. Oil or butter a cookie sheet, or cut a piece of parchment paper to fit it. Have ready a biscuit cutter or jar 3 inches in diameter, and ½ cup flour in a bowl. Whisk two eggs with a pinch of salt in another bowl, to glaze the cut biscuits, and dig out a pastry brush if you have one.
Make a well in the middle of the sifted flour, pour in the milk-oil mixture, and sprinkle the raisins on top. Blend the flour into the liquid-raisin mixture with a wooden spoon, working from the inside out and mixing as lightly as you can. The dough will come together easily and quickly. Turn it out onto a large sheet of waxed paper and cover it with another, so that you can pat it into a rough rectangle an inch high; it should measure roughly 8 by 10 inches.
Remove the top sheet, scatter the frozen raspberries over half of the rectangle (try to break them into individual berries as you work), and fold over the other half. Cover the waxed paper again and roll or pat the rectangle until it is an even 1 ½ inches high. Remove the top sheet and cut the dough into rounds with clean strokes, dipping the cutter into flour between cuts. Transfer the biscuits onto the baking sheet with a table knife or offset spatula dipped in flour, spacing them 1 ½ to 2 inches apart. You can pat the scraps together into messy rounds and even them out with the cutter. Glaze the tops with the beaten egg mixture, and slide the finished tray into the refrigerator for 15 minutes or the freezer for 10 minutes; glaze the scones again.
Put the tray in the oven and turn the temperature down to 425 degrees. Bake for 15 minutes for 2 1/2 – inch biscuits or 18 minutes for 3 –inch biscuits. Resist the urge to cook them longer, even though the tops only start to get shiny and brown in the last few minutes of baking. Cool them on the baking sheet for 10 minutes, which will allow the bottoms to finish browning, and then transfer to a cooling rack.