• Note before starting: These cookies need to chill for at least 24 hours (preferably 48 hours) once shaped and filled. This is vital to the thick, chewy texture and deep, rich flavor of the cookies, and cannot be skipped or rushed by placing in the freezer. It's worth the wait, I promise.

Brown the Butter:

• Place the cubed butter in a medium stainless steel saucepan or skillet. Over a medium heat, melt the butter, and allow it to brown. This will take approximately 5-8 minutes. Stir occasionally. The butter will foam and become a bit hard to see the bottom – that's okay. Keep an eye on it until the butter is golden brown and there are noticeable deeply brown bits at the bottom.
• Remove from heat and transfer to a large heatproof mixing bowl, scraping in all the little brown bits at the bottom of the pan for maximum flavor.
• Add the brown sugar and granulated sugar to the hot butter. Whisk carefully and combine well. Set aside to cool to almost room temperature (about 20 minutes).

Make the Cookies:

• Add the sourdough discard, eggs, egg yolk, and vanilla to the bowl. Whisk well to combine.
• Add the all-purpose flour, baking powder, baking soda, and salt to the bowl. Using a rubber spatula, mix until almost combined. Ensure you incorporate the very bottom of the bowl.
• Add the chopped chocolate. Stir until just combined, being careful not to over-mix.
• Cover the bowl with plastic wrap and place in the fridge for about 30 minutes, or until firm enough to roll into balls.
• Meanwhile, prepare a large, rimmed baking sheet with parchment paper.

Shape and Chill the Cookies:

• Once firmed, use a large 3-Tablespoon cookie scoop to portion the cookie dough into balls. Place on the prepared baking pan. Cover well with plastic wrap and place in the fridge to chill for at least 24 hours (I prefer to chill for 48 hours for the best flavor and texture).

Fill the Cookie Dough Balls:

• Using your fingers, create a well in the middle of a cookie dough ball. Pipe or spoon 1 teaspoon of jam inside each cookie dough ball. Gently wrap the edges of the dough around the jam, leaving a little visible on top in the middle. Repeat with the remaining cookie dough balls. Chill the filled dough balls while the oven preheats.

Bake the Cookies:

• When ready to bake, preheat oven to 375°F / 190°C. Line light-colored rimmed baking sheets with parchment paper.
• Bake cookies on prepared pans, spaced at least 3 inches apart, for 9 to 12 minutes, or until lightly golden brown at the edges. Dot the tops of each cookie with additional chocolate and sprinkle with flaky sea salt, if desired.