Whisk together the flour, baking powder, baking soda, and salt in a medium bowl until evenly combined.

In a large bowl, beat the butter and sugar for 2–3 minutes until pale and fluffy.

Mix in the sourdough starter, egg yolk, lemon juice, lemon zest, vanilla, and optional food coloring until smooth.

Gradually add the dry ingredients, mixing just until a soft dough forms. Do not overmix.

Cover and refrigerate for at least 2 hours or up to 3 days to firm up the dough and deepen flavor.

Preheat the oven to 350°F and line a baking sheet with parchment paper. Scoop dough into 1-inch balls.

Roll each ball generously in powdered sugar for a thick coating.

Place cookies 2 inches apart and bake for 11–13 minutes, until edges are set and centers remain soft.

Let cool for 5 minutes before serving; they will firm up slightly while staying tender inside.