Preheat oven: Preheat your oven to 350°F (175°C).
Prepare flax egg: Combine 1 tablespoon ground flaxseed with 2.5 tablespoon water. Let sit for 5 minutes to thicken.
Mix dry ingredients: In a large bowl, combine oats, cinnamon, baking powder, and salt.
Mix wet ingredients: In a separate bowl, whisk together milk, maple syrup, melted coconut oil, vanilla extract, and the prepared flax egg.
Combine: Pour the wet ingredients into the dry ingredients. Mix until just combined.
Add extras: Fold in the protein powder (if using), frozen blueberries, and chocolate chips.
Bake: Pour the mixture into a greased or parchment-lined baking dish. Bake for 35 minutes, or until the edges are golden brown and a knife inserted into the centre comes out clean.
Cool and serve: Allow to cool slightly before serving. Enjoy warm or store for later.