Making the Cookie Dough

1. In a large mixing bowl, whisk together the flour, baking soda, corn starch, salt, and chocolate chips. Set aside.

2. In another bowl, mix the sourdough starter, melted butter, brown sugar, white sugar, egg, egg yolk, and vanilla extract. Stir until the mixture is smooth and glossy, like runny caramel.

3. Pour the wet ingredients into the dry mixture and stir until a soft cookie dough forms.

4. Cover the bowl with plastic wrap and refrigerate for 12 to 24 hours to let the flavors develop.

Baking the Cookies

1. When ready to bake, preheat your oven to 165°C (330°F) and line two baking sheets with parchment paper.

2. Scoop out cookie dough into 65g portions (or your preferred size) and place on the prepared trays, leaving space between each.

3. Bake for 10 minutes. The cookies will be partially baked—this is when you’ll add the toppings.

Adding the S’mores Toppings

1. Carefully take the cookies out of the oven. Gently flatten the tops with a spoon.

2. Press in pieces of Hershey’s chocolate, marshmallow halves, graham cracker crumbs, and a pinch of flaky sea salt. Add as much or as little as you like, but press them in slightly so they bake into the dough.

3. Return the cookies to the oven and bake for another 5 to 8 minutes, depending on how soft or crisp you like them. Watch closely—marshmallows can melt quickly.

Cooling the Cookies

1. Once the edges are golden brown and the tops look set, remove the trays from the oven.

2. Let the cookies cool on the tray for about 10 minutes. They’ll firm up as they sit.

3. After 10 minutes, transfer the cookies to a wire rack to cool completely.