Line an 8 x 8-inch baking dish with parchment paper and set it aside.
In a large bowl, whisk together the oat flour, coconut flour, vanilla protein powder, sugar (if using), and cinnamon until evenly combined.
In a microwave-safe bowl, warm the almond butter and brown rice syrup together until they soften and can be stirred to a smooth consistency.
Pour the warmed wet mixture into the dry ingredients and stir to combine, then add the mashed banana and mix until incorporated.
Fold in the chopped nuts if using, then add the milk one tablespoon at a time, stirring after each addition, until a thick, firm batter forms.
Transfer the mixture to the prepared baking dish and press it down firmly into an even layer.
Refrigerate for at least 30 minutes to set, then slice into 12 bars.