Preheat the oven to 350°F (175°C).

Grease the baking dish with coconut oil or butter.

In a large mixing bowl, combine rolled oats, baking powder, cinnamon, and salt.

In another bowl, whisk together milk, honey or maple syrup, melted coconut oil, and vanilla extract.

Pour the wet mixture into the dry ingredients and stir until just combined.

Gently fold in your choice of mix-ins.

Transfer the mixture to the prepared baking dish and spread it evenly.

Bake for 30 minutes or until the top is golden brown.

Let it cool for a few minutes before slicing into squares.