Melt the butter: 1 minute in the microwave or in a pan, on the stove top. Transfer to a bowl; cool completely before using.

Add the flour, cinnamon, baking soda, baking powder and fine sea salt to a large bowl; whisk to combine.

In a separate small bowl: whisk the cooled melted butter, brown sugar, granulated sugar, sourdough starter, egg, egg yolk and vanilla extract.

Add the wet ingredients to the dry ingredients; mix well to combine.

Fold in the oats, raisins and walnuts (if using). Scrape down the sides of the bowl with a rubber spatula. The dough will be sticky.

Cover the bowl and chill the dough for a minimum of 1 hour (or overnight). Please refer to Notes & Tips in the top section of this recipe for additional understanding.

Before you are ready to bake, check the texture of the cookie batter. Roll a small portion of dough in-between your palms; it should hold together with a tacky (not sticky) texture. If the dough feels too wet and sticks to your hands, add a tablespoon of flour, mix well, and chill for another 20 minutes.

Preheat the oven to 350 F. Line a baking sheet with parchment paper.

Scoop the dough in heaping tablespoon-sized portions onto your lined sheet pan. Use a cookie scoop or a regular tablespoon.

Bake for 10-11 minutes or light golden brown.

Remove from the oven. Cool the cookies on the tray for approximately 5 minutes, and then transfer to a wire rack to finish cooling.