Preheat the oven to 350°F and line two baking sheets with parchment paper.
Combine the flour, baking soda, baking powder, salt, and cinnamon in a large bowl. In a separate bowl, whisk together the melted coconut oil, sugar, whole egg, egg yolk, and vanilla, whisking vigorously.
Stir the wet ingredients into the dry ingredients. Stir in the oats, raisins, and walnuts, if using, folding into a tight batter. Set the dough aside for 20 minutes while the oven preheats. (Note: if your dough seems too wet to become scoop-able, chill it in the fridge for this 20 minutes and it'll firm up). If your dough is too crumbly, stir in 2 to 3 tablespoons water.
Scoop into 20 tablespoon-sized balls and roll lightly in barely damp hands to make them round. Spread out onto the prepared baking sheets and bake until puffed, golden, and a touch underbaked-looking, 10 to 11 minutes. Let cool on the pans for 5 minutes before transferring to a wire rack to cool completely.
Find it online at https://www.loveandlemons.com/oatmeal-cookies/