Prepare Pan: Line an 8x8-inch or 9x9-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. This is crucial for no-bake bars.
Process Oats (Optional, for smoother texture): If you prefer a smoother brownie texture, pulse the rolled oats in a food processor a few times until they resemble a coarse flour. If you like a more rustic, slightly chewy texture, use whole oats.
Combine Dry Ingredients: In a large mixing bowl, whisk together the (processed) oats, unsweetened cocoa powder, and salt.
Combine Wet Ingredients: In a separate medium bowl, or in the microwave, combine the peanut butter, maple syrup (or honey), and milk. Microwave for 30-60 seconds, or gently heat on the stovetop, until the mixture is warm and runny enough to stir smoothly. Stir in the vanilla extract.
Combine Wet and Dry: Pour the wet mixture into the dry oat mixture. Stir well with a sturdy spoon or spatula until everything is thoroughly combined and a thick, fudgy dough forms. If adding mini chocolate chips, fold them in now.
Press into Pan: Transfer the brownie mixture to the prepared pan. Using a spatula, the back of a spoon, or your clean hands, press the mixture down firmly and evenly into the pan. Make sure it's packed tightly to ensure the bars hold together.
Prepare Ganache (Optional): If making the ganache topping, combine chocolate chips and milk (and optional butter/coconut oil) in a microwave-safe bowl. Microwave in 30-second intervals, stirring well after each, until smooth and melted. Alternatively, melt in a double boiler.
Add Topping: Pour the melted ganache evenly over the pressed brownie base. Spread it gently with a spatula to cover the entire surface.
Chill: Place the pan in the refrigerator for at least 1-2 hours (or freezer for 30 minutes) until the bars are firm and the ganache (if used) is set.
Slice and Serve: Once firm, use the parchment paper overhang to lift the entire slab out of the pan. Place it on a cutting board and slice into squares or rectangles using a sharp knife. For cleaner cuts, you can run your knife under hot water and wipe it clean between cuts.
Storage: Store the no-bake brownie bars in an airtight container in the refrigerator for up to 1 week. They can also be frozen for up to 1-2 months.