In a medium saucepan, melt the butter over medium heat. Avoid using a dark pan as you will struggle to see the caramelization of the butter and run the risk of burning it.

Once melted, let it cook further for around 3-4 minutes, stirring occasionally until the milk solids in the butter have caramelized into a golden amber-brown color and the butter has a nutty aroma.

Pour the melted butter into a large heat-proof bowl, including all the caramelized brown bits at the bottom of the pan, and allow it to cool down for 10 minutes.

In a small separate bowl whisk together the flour, oats, cinnamon, salt, and baking soda and set it aside.

Once the butter has cooled, whisk in the brown sugar, granulated sugar, sourdough starter egg, and vanilla.

Stir the dry ingredients into the wet ingredients using a rubber spatula or wooden spoon, followed by the chocolate chips.

Cover the dough and chill for at least 2 hours or overnight. For easier rolling, you can chill the dough for 30 minutes until slightly stiffened, then roll it into balls and chill the cookie dough balls for the remainder of the time.

When baking, line a cookie sheet with parchment paper. Preheat the oven to 350F/180C.

Roll a tablespoon and a half of dough per cookie into a ball. Place the cookie dough balls around 2 inches/ 5 cm apart on the prepared baking sheets.

Bake the cookies for 10-12 minutes until the cookies are golden brown around the edges but still a little soft. It's best to underbake them than over-bake them. Underbaking gives a great chewy texture, while over-baking can make them too cakey.

Let the cookies cool for 5 minutes on the baking sheet before placing them on a wire rack to cool further.