Whisk together flour, sugar, salt, and baking powder in a large bowl.
Grate in the frozen butter and cut it into the flour until the mixture resembles coarse crumbs with some pea-sized pieces. Stir in the raisins to coat.
In a separate bowl, whisk together sourdough discard, egg, and vanilla until smooth, then add to the dry ingredients.
Slowly pour in the sour milk, mixing just until the dough comes together into a shaggy ball (you may not need all the liquid).
Turn the dough onto a lightly floured surface and gently pat into a 1–1½ inch thick circle.
Cut into 8 wedges and place on a parchment-lined baking tray, spacing apart. Chill for 15 minutes while preheating the oven to 400°F (204°C).
Brush the tops with sour milk, sprinkle with brown sugar, and bake for 10–15 minutes until risen and golden.
Cool for at least 30 minutes before serving. Serve with butter and jam.