Preheat your oven to 350°F (180°C) and line a large baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar until light and fluffy—about 2–3 minutes.
Beat in the eggs one at a time, then mix in the lemon zest, lemon juice, and vanilla extract.
Slowly add in the dry ingredients and mix just until a dough forms. It’ll be soft and slightly sticky, but that’s perfect. Cover the dough and refrigerate for 30 minutes to allow the dough to firm up, making it easier to handle.
Once the dough has firmed up in the fridge, divide it into equal halves. On a lightly floured surface, shape each half into a log about 12 inches long and 2–3 inches wide. Carefully place each log on your prepared baking sheet. Leave space between them to allow for spreading.
Bake for 25 minutes, or until the logs are firm and lightly golden. Remove them from the oven and allow to cool for 10 minutes. Reduce the oven temperature to 300°F.
Once cooled slightly, carefully transfer the logs to a cutting board and use a sharp serrated knife to slice into ½- to ¾-inch wide pieces.
Lay the biscotti slices cut side down on the baking sheet and return to the oven. Bake at 300°F for another 10 minutes, flip them over, and bake the other side for another 8–10 minutes, until golden and crisp.
Let the biscotti cool completely on a wire rack. While they cool, whisk together the powdered sugar, lemon juice, and zest until you get a smooth drizzle.
Once the biscotti are completely cooled, drizzle the icing over the tops using a spoon or piping bag. Let the icing set before storing or serving. ENJOY!