Preheat your oven to 350 degrees Fahrenheit. Line two baking sheets with parchment paper or silicone mats.
In a large bowl or the bowl of a stand mixer, cream the softened butter, brown sugar, and cane sugar until light and fluffy.
Mix in the egg, vanilla extract, and sourdough discard. Stir until smooth and well combined.
In a separate bowl, whisk together flour, oats, cinnamon, salt, and baking soda. Add the dry ingredients to the wet ingredients and stir just until combined.
Mix in raisins with a wooden spoon or spatula.
Drop heaping tablespoons of dough onto the prepared cookie sheet, spacing them a few inches apart. Bake for 10-12 minutes, or until the edges are golden brown and the centers are just set.
Let the cookies cool on the baking sheet for 10 minutes before transferring to a wire rack. Serve warm or store in an airtight container for up to five days.
Notes
If your dough feels sticky, chill it for 10 to 15 minutes before handling.
For softer cookies, bake for less time. For crispier edges, bake a minute or two longer.
Use room temperature butter and eggs for best results.
You can soak raisins in warm water for 10 minutes before adding them to the batter to make them extra plump.