Preheat your oven to 350°F (175°C). This ensures that your bars bake evenly.
Line a 9x13 inch baking pan with parchment paper, leaving some overhang on the sides.
In a large mixing bowl, whisk together the melted butter, light brown sugar, and granulated sugar until smooth and creamy. Add in the eggs, egg yolk, milk, and vanilla extract, mixing until fully combined.
In another bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon.
Gradually add the dry mixture to the wet mixture, stirring until just combined.
Gently fold in the rolled oats and your choice of chopped dark chocolate or other mix-ins.
Spread the dough evenly into the prepared baking pan, pressing it down gently with a spatula or your hands.
Bake in the preheated oven for about 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out mostly clean.
Allow the bars to cool in the pan for about 10 minutes, then use the parchment paper to lift them out onto a wire rack to cool completely. Once cool, slice into squares or bars.
