Preheat oven to 375’F
Grease an 8x8 baking dish (you can use smaller or larger but cooking times will vary. For a smaller pan, you will have to bake longer to ensure it's fully cooked in the center).
Combine oats, raisins, walnuts, baking powder, cinnamon, chia seeds and salt in a medium mixing bowl
Combine brown sugar, milk, egg and vanilla in a separate mixing bowl
Add wet to dry mixture, folding gently to combine. Let sit for 2 minutes to allow chia seeds to soak up some of the moisture
Pour into greased baking dish using a spatula to spread the mixture evenly throughout the dish
Bake for 30-40 minutes until golden brown (because every oven is different, I recommend plunging a knife or cake tester into the center of the dish to test if it's done, if it comes out clean or just a bit wet, it’s done).
Allow to cool completely before slicing. You don't HAVE to wait, but note that the squares may fall apart a bit if you're scooping it out of the dish right when it comes from the oven.