About 30 minutes before making the scones, put the butter in a bowl and place in the freezer. Preheat the oven to 400 (200C) degrees with the racks arranged in the center.
Line two rimmed baking sheets with parchment paper.
Combine the flour, baking powder, baking soda, salt and ⅓ cup of the sugar in a food processor work bowl or bowl of a standing mixer fitted with the paddle. Pulse the dry ingredients or mix on low until incorporated.
Add the butter and pulse a few times until the mixture is pale yellow and the texture of fine meal. If using a standing mixer, mix on low speed until the mixture looks right, which will take a few minutes.
Add the vanilla and buttermilk and pulse/mix just until a dough forms - don't overdo it or the dough will toughen.
Dump the mixture onto a lightly floured surface. Gently knead the dough together to form a ball, then pat it into a circle slightly thicker than 1/2-inch. Cut out circles as close together as possible and arrange 1-inch apart on the baking sheets. Gather the scraps, gently patting and pressing together and cut out the remaining dough.
Brush the tops of the scones with the egg white, then sprinkle lightly with the remaining teaspoon sugar.
Bake about 15 minutes, until the scones are lightly browned. Serve warm, split in half and spread with mascarpone cheese, jam, and some strawberries.
The scones can be made ahead and frozen. Mix the dough, cut them out and freeze on trays until frozen, then transfer to a freezer storage container. When ready to bake, pull them out of the freezer and follow baking directions.