Add the egg, vanilla extract, and sourdough discard to the butter-sugar mixture and mix on low speed until fully combined. Scrape down the edges of the bowl as needed.

In a medium bowl, whisk together the flour, salt, and cornstarch.

With the mixer on low speed, add the dry ingredients about 1/4 cup at a time until all of the dry ingredients are mixed into the wet ingredients. Do not over mix. The dough will be slightly sticky.

Cover the bowl with a dish towel and place in the refrigerator for one hour.

After about 30 minutes and while the dough is still chilling in the refrigerator, preheat your oven to 350 degrees and line a baking sheet with parchment paper.

After the dough has chilled, use a 2 tablespoon cookie scoop to scoop out balls of dough. Gently roll the dough into a ball and roll in sugar.

Place the cookies onto the parchment paper-lined cookie sheet about 3 inches apart.

Using a rounded teaspoon or the end of a small spoon, make an indent in each of the cookie dough balls.

Fill the indent of each cookie with 1/2 teaspoon of the raspberry preserves.

Place the baking sheet in your preheated oven and bake for 14 to 16 minutes or until the edges of the cookies are lightly golden and set.