Preheat the oven to 350°F. Line two baking sheets with parchment paper and set them aside.

Using a stand mixer, combine the butter and sugar in the bowl and beat until light and fluffy. Scrape the sides of the bowl, then add the egg and mix to combine. With the mixer running, add the sourdough starter discard, ricotta, lime zest, and vanilla, and mix until fully incorporated. If the mixture becomes lumpy, this is normal.

Meanwhile, whisk the flour, baking powder, and salt together in a separate bowl. On low speed, gradually add the dry ingredients and mix until just combined. Scrape down the sides of the bowl again, making sure no flour remains at the bottom.

Cover the bowl with plastic wrap and refrigerate the dough for 30 minutes to 1 hour. Then, scoop the dough onto your baking sheets using a mini ice cream scoop. You can leave the shape as is or roll each scoop into a ball and gently flatten.

Place both baking sheets in the oven and bake for 15 to 17 minutes. Rotate the sheets and swap racks at the halfway mark for even browning. Once done, the cookies should be soft yellow with golden bottoms and springy to the touch. Transfer them to a wire rack.

For the lime glaze, sift the powdered sugar into a small bowl. Add the juice of 1 lime and whisk until smooth. If the glaze is too thick, add more lime juice until it reaches the desired consistency.

Once the cookies have completely cooled, drizzle the lime glaze over the top. If desired, sprinkle with grated lime zest. Enjoy!