Preheat your oven to 350°F and line a baking sheet with parchment paper.
In a light-colored skillet or small saucepan over medium-low heat, add the butter and melt it for 8-10 minutes, until it smells like toffee and you can see little brown bits at the bottom of the pan. Once you start to smell it, take the butter off the heat to cool.
Once the butter is cooled but still liquid, add it to a large mixing bowl along with the granulated sugar and dark brown sugar.
Add the sourdough discard, egg yolks, and vanilla. Whisk together until well combined.
Sift in the flour, baking soda, and salt, stirring with a wooden spoon or rubber spatula until a thick dough forms (if you don't have a sifter, whisk dry ingredients together in a separate bowl). Use a rubber spatula to fold in the chocolate chips.
Use a 1 ½ tablespoon cookie scoop to scoop mounds of dough onto your prepared baking sheet.
Bake for 9-11 minutes until the cookies are set, and the edges are light golden brown. For gooey cookies, don't over-bake.
Immediately after they come out of the oven, use the bottom of a measuring cup to press them down and flatten them slightly. Sprinkle with flaky sea salt if using.
Optional: for perfectly round cookies, use a biscuit cutter or drinking glass to swirl the cookies around while they’re still hot. This will scoot the edges into perfect circles.
Allow to cool on cookie sheet for 5 minutes, before removing to a wire rack to cool completely.
Notes
Use a sourdough starter that has been recently fed for a less sour flavor.
Use a cookie scoop so the cookies are all the same size.
When browning your butter, keep the heat at medium-low and use a light colored pan, if possible. The light colored pan helps you see the butter solids starting to brown, so you don’t risk them burning.
Bake the cookies just until they’re set but still a little soft in the center. Do not over-bake!
Measure your flour correctly by fluffing the flour a bit and then spooning it into the measuring cup. Use the handle of the spoon to scrape the excess back into the container so you have a completely level top. Or, use a kitchen scale to measure your flour. You’ll need 210g of all-purpose flour.
You can let these cookies long ferment for up to 2 days in the fridge, but make sure they’re covered so the dough doesn’t dry out.
Make them bigger: These cookies are smaller, but if you want a big bakery-style cookie, use a 2 or 3 Tablespoon cookie scoop and bake them for 12-14 minutes.
Sifting your dry ingredients ensures everything incorporates evenly. If you don’t have a sifter, you can just put the dry ingredients into a separate bowl and whisk them together before adding them to the wet ingredients.
Storage: Store at room temperature for up to 5 days in a covered container. Or freeze for up to 4 months.How to Freeze Dough: To freeze the dough, scoop the cookie dough balls onto a parchment lined baking tray and freeze for 2-3 hours (or overnight) until solid. Transfer the cookie dough balls into a labeled and dated freezer bag, and they’ll stay fresh for 3-4 months. Follow baking instructions above, adding a few minutes to cooking time.