1. Adjust oven rack to middle in a countertop oven or lower middle position in oven and preheat to 400 degrees.
2. In the bowl of your stand mixer, combine flour, 1/3 cup sugar, baking powder, baking soda, and salt. Grate butter into flour mixture on the large holes of a box grater; use your fingers to work in butter (mixture should resemble coarse meal), then stir in lemon zest.
3. In a small bowl, whish sour cream, lemon juice, and egg until smooth.
Using a fork, stir sour cream mixture into flour mixture until large clumps of dough form. Use your hands to press the dough against the bowl into a ball. (The dough will be sticky in places, and there may not seem to be enough liquid at first, but as your press, the dough will come together).
5. Place on a lightly floured surface and pat into a 7- to 8-inch circle about 3/4-inch thick. Sprinkle with remaining 1 teaspoon of sugar. Use a sharp knife to cut into eight triangles; place on a cookie sheet (preferably lined with parchment paper), about one inch apart. Bake until golden, about 15 to 17 minutes. Cool for 5 minutes and serve warm or at room temperature.
Recipe provided by Breville