Preheat the oven to 400°F and line a baking sheet with parchment paper. Combine the flour, sugar, baking powder, and salt in a large mixing bowl. Incorporate the butter into the dry mixture using your hands until the texture resembles coarse, damp sand. In a separate bowl, whisk together the egg, sour cream, milk, lemon juice, and vanilla extract. Gently fold the wet ingredients into the flour mixture until just blended. Transfer the dough onto a floured surface and form it into two circles, each approximately ½ inch thick. Position one circle on a cutting board and evenly spread the strawberry jam over it, leaving a ½-inch border around the edges. Place the second circle over the jam-covered base and press the edges to seal. With a sharp knife or pizza cutter, divide the layered dough into eight wedges. Arrange the scones on the prepared baking sheet, spacing them 2–3 inches apart. Lightly brush the tops with milk and sprinkle with sparkling sugar if using. Bake for 20–22 minutes until golden brown. Allow the scones to cool fully on a wire rack.