Cream butter and sugar in large bowl with mixer until light and fluffy for 3-5 minutes.
Add in sourdough starter, vanilla, and egg and mix until incorporated.
Add flour, baking powder, and salt into wet until just incorporated. Scrape sides of bowl to make sure everything is combined.
Cover dough with plastic and refrigerate for at least a few hours or up to 24 hours for long fermentation if desired.
In a clean small bowl, add ¼ cup granulated sugar.
Use a tablespoon and scoop out the dough and form into a tight little ball.
Roll ball into the sugar bowl until well covered.
When cookies are ready for baking, preheat oven to 350°F.
Place ball on tray lined with parchment paper and using a spoon or thumb, press an indentation in center until cookie is rounded out. Do this for remainder of dough.
Add a teaspoon or so of jam in the indent for each cookie.
Bake for 12-15 minutes, or until cookies are set.
Remove from oven and allow to cool for 10 minutes on baking sheet before transferring to a rack to cool completely.