Preheat your oven to 350°F (175°C).
Lightly grease an 8x8-inch baking dish.
In a large mixing bowl, combine the rolled oats, protein powder, baking powder, cinnamon, and salt.
In another bowl, whisk together the almond milk, honey or maple syrup, Greek yogurt, vanilla extract, and egg until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Fold in the mixed berries and nuts if using.
Pour the mixture into the prepared baking dish and spread evenly.
Bake for 25-30 minutes until golden brown and a toothpick comes out clean.
Let it cool for about 10 minutes before slicing into squares.