Preheat the oven to 300°F, and coat an 8”-square pan with nonstick cooking spray.
In a medium bowl, whisk together the oats, flour, baking powder, cinnamon, nutmeg, and salt. In a separate bowl, whisk together the butter or coconut oil, egg white, and vanilla. Stir in the maple syrup until completely incorporated. Stir in the milk. Add in the oat mixture, and stir until just incorporated. If using any optional mix-ins, gently fold them in.
Spread the mixture into the prepared pan. Bake at 300°F for 9-12 minutes. Cool completely to room temperature in the pan. Allow them to rest for an additional 6+ hours once at room temperature for the best soft and chewy texture before slicing into squares.
