Preheat the oven to 350 degrees then spray an 8x8" baking pan with nonstick spray and set it aside.
Add 1 cup old fashioned oats to a food processor or high powered blender then process until the oats have turned into flour. Set aside. Alternatively you can use a scant cup of store-bought oat flour.
To a large mixing bowl, add the milk, mashed banana, maple syrup, melted coconut oil, egg, and vanilla extract then whisk to combine. Note: the coconut oil might solidify a bit as it's whisked with the cold ingredients. That's ok - it'll melt in the oven.
To the bowl, add the oat flour, remaining 1-1/2 cups oats, cinnamon, baking powder, and salt then stir with a spatula to combine. Fold in the chopped pecans then pour the batter into the prepared baking pan.
Bake for 35-40 minutes, or until the edges of the bars are golden brown and the center has completely set. Check on the bars at the 25 minute mark - if the edges are browning too quickly, gently a piece of foil on top of the baking pan and continue baking.
Cool then slice the bars and store in the refrigerator for up to 5 days. Or, individually wrap cooled bars in saran wrap then freeze for up to 3 months. Thaw in the refrigerator.
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Notes
You can swap the egg for a flax egg - just plan on tacking on extra baking time to get to a point where the center has completely set.