Preheat your oven to 350°F (175°C). Line your prepared baking sheet with parchment paper.
In a large mixing bowl, use a wooden spoon or a hand mixer to cream together the unsalted butter, brown sugar, and white sugar until light and fluffy.
Beat in the egg yolk and vanilla extract until well combined. Stir in the sourdough starter discard.
In a separate bowl, whisk together the purpose flour, baking soda, baking powder, and ground cinnamon.
Gradually mix the dry ingredients into the wet ingredients until combined. Fold in the old-fashioned oats, juicy raisins, and walnuts chips (if using).
Use a cookie scoop to drop cookie dough balls onto the parchment-lined baking sheet, leaving inches of space between each ball.
Bake for 10-12 minutes, or until the edges of the cookies are golden brown. The centers should be slightly soft but will firm up as they cool.
Allow the cookies to cool on the cooling rack or wire rack for a few minutes before transferring them to an airtight container.