1. Add softened butter to a large bowl. Beat with an electric mixer on medium speed until creamy, about 30 seconds. 
   170 grams unsalted butter, room temperature
2. To the butter, add the white sugar and tightly packed light brown sugar. Cream on medium speed for an additional 30 seconds until combined. 
   75 grams white sugar, 75 grams light brown sugar
3. Then, add the egg yolk, sourdough starter discard, and vanilla extract and mix until combined.
   1 egg yolk, 50 grams sourdough discard, 4 grams vanilla extract
4. In a separate bowl, whisk together the flour, cornstarch, and salt. 
   280 grams all-purpose flour, 6 grams cornstarch, 3 grams salt
5. With your mixer on low speed, gradually add the dry ingredients to the butter mixture. Occasionally you will need to stop and scrape the sides of the bowl. The cookie dough will look dry and crumbly, but keep mixing through this, everything will come together. 
6. With a cookie scoop or hand, scoop about 1 Tablespoon worth of cookie dough and roll it into a tight and smooth ball. It is important that there are no air pockets in the dough, otherwise, this can lead to cracks in the exterior of the cookie dough.

*In-between batches, I like to store the cookie dough or shaped cookie dough balls in the fridge so the butter doesn't melt.

1. If desired, roll the balls of dough in granulated sugar. This gives the cookies a sweet and crisp outer layer. It also helps to prevent your teaspoon or thumb from sticking to the dough when you are making your indents. 
2. With a the back of small teaspoon or your thumb, make a small indent in the center of each cookie. If the cookie dough cracks, reroll and try again.
3. Repeat with the rest of the cookie dough. 
4. With a small spoon, ladle your favorite jam or preserves into the indent of your cookie.
   2/3 cup of your favorite jam or preserve
5. When you pull the jam/preserves from the jar, it will be a little firm, so I like to scoop it into a small bowl and heat it in the microwave for 5-10 seconds. This softens the fruit filling and makes it easier to spoon onto your cookies. *A neater option would be to add your jam to a piping bag. You choose!
6. Line the baking sheet with parchment paper. 
7. Then add your sourdough thumbprint cookies to your prepared cookie sheet, spacing them about 1-2 inches apart. 
8. Bake your cookies in a 350-degree F oven for 15-17 minutes, or until the top is set and the edges of the cookies are a light golden brown. 
9. Remove the cookies from the oven and place them on a wire rack to cool. Enjoy!