Combine flour, sugar, baking powder, and salt to a food processor and pulse just until mixed together. Add pieces of butter and pulse again until small crumbs form.
2 cups all purpose flour, ½ cup granulated sugar, 1 teaspoon baking powder, ¼ teaspoon kosher salt, ½ cup cold unsalted butter, cut into pea-sized pieces
In a separate bowl, whisk together egg, egg yolk, and lemon zest.
1 whole egg, plus 1 yolk, 1 teaspoon lemon zest
Then, with the food processor running, slowly add the egg-zest mixture through the feed tube. Mix, then pulse until dough forms around the blades. Pinch dough between your fingers...if it sticks together easily, it’s done.
Separate dough into two balls. Press each ball into a flat disc, wrap in plastic wrap, and refrigerate at least 30 minutes. If refrigerated for over 1 hour, let it rest at room temperature for 30 minutes before attempting to roll it out.