Preheat the oven to 350°F and line the bottom of an 8 x 8 inch baking pan with two sheets of parchment paper, extending the sheets in opposite directions and leaving a bit of overhang, which you'll use later on to remove the bars from the pan.
Stir the milk and flaxseeds together in a small bowl or container. Let the mixture sit for about five minutes, in order to give the seeds time to soak up the milk.
While the flaxseed mixture sits, place the banana into a large bowl. Mash it well using a fork or potato masher.
Stir the flax mixture, peanut butter, maple syrup, and vanilla in with the banana.
Add the oats to the bowl with the banana mixture, then sprinkle the cinnamon, banking powder, and salt over the oats. Stir the ingredients together until they're well combined.
Stir the raisins, walnuts, and chocolate chips into the oat mixture.
Transfer the mixture to the prepared baking pan, flattening and smoothing out the top as much as you can.
Place the pan into the oven and bake the bars until the top feels set, about 20 minutes.
Place the baking pan on a cooling rack and let the bars cool completely.
Once the bars are cool, use the excess parchment paper to carefully lift the bars from the pan. Cut them into squares.
Serve the bars immediately, or seal them up and store them at room temperature for up to three days, in the fridge for up to five days, or in the freezer for up to three months.