1. Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.

2. In a large bowl, beat softened butter with granulated and brown sugars until light and fluffy, about 2–3 minutes.

3. Mix in egg yolk, vanilla extract, and sourdough discard until combined.

4. Whisk flour, baking powder, and salt in a separate bowl; gradually add to wet mixture until a soft dough forms.

5. Cover dough and refrigerate for 30–45 minutes to chill.

6. Roll chilled dough into 1-inch balls and place 2 inches apart on prepared sheet.

7. Press a shallow well into the center of each ball with your thumb or the back of a spoon.

8. Bake 9–11 minutes until edges are lightly golden and centers remain pale, then let cool on the sheet for 5 minutes before transferring to a rack.

9. Whisk sour cream and vanilla in a bowl, then gradually whisk in powdered sugar until smooth and spreadable.

10. Once cookies are fully cooled, fill each thumbprint with sour cream frosting.

11. Decorate with sprinkles or jam if desired and let frosting set before serving.