In a large bowl, whisk together flour, sugar, baking powder, and salt. Using a pastry blender or 2 forks, cut in cold butter until it resembles coarse crumbs. Add sultanas, tossing until combined. Add 1 egg, stirring with a fork just until combined. Stir in cold butter milk just until dry ingredients are moistened. Working gently, bring mixture together with hands until a dough forms. Gently knead 3 to 4 times.
Turn out dough onto a lightly floured surface. Cover and let rest at room temperature for 10 minutes.
Preheat oven to 375°. Line a rimmed baking sheet with parchment paper.
Using a rolling pin, roll out dough to a 1-inch thickness. Using a 2¼-inch round cutter, cut 12 scones from dough, rerolling scraps as necessary. Place scones 1 inch apart on prepared baking sheet. Let stand at room temperature for 30 minutes.
Brush tops of scones with remaining 1 egg.
Bake until tops of scones are golden brown and a wooden pick inserted in centers comes out clean, 15 to 20 minutes. Serve warm.
Recipe by TeaTime Magazine at https://teatimemagazine.com/sultana-buttermilk-scones-recipe/