Prepare Your Pan: Line a 9x13 inch (or similar size) baking dish with aluminium foil or baking parchment, leaving an overhang on the sides. This overhang will act as a sling, making it much easier to lift the set bars out of the dish later. Lightly grease the foil or parchment with a little butter or coconut oil.

Melt the Wet Ingredients: In a medium saucepan set over a low heat, combine the peanut butter, honey (or maple syrup), unsalted butter (or coconut oil), and salt. Stir continuously until the mixture is completely melted and smooth, forming a glossy, cohesive sauce. You'll notice a rich, nutty aroma filling your kitchen, and the mixture will be a uniform, creamy caramel colour. Remove from the heat.

Add Vanilla and Combine: Stir in the vanilla extract until fully incorporated into the warm peanut butter mixture. Then, pour this wet mixture over the rolled oats (and optional flaxseed meal or chia seeds if using) in a large mixing bowl. Fold gently with a spatula until all the oats are thoroughly coated and no dry patches remain, creating a sticky, uniform oat mixture.

Press into Pan: Transfer the oat mixture into your prepared baking dish. Using the back of a spoon or your hands (lightly dampened to prevent sticking), press the mixture down firmly and evenly into the pan. The surface should be smooth and compact, ensuring the bars hold together well.

Add Chocolate Topping: Sprinkle the dark chocolate chips and any optional milk chocolate chips evenly over the pressed oat mixture. Gently press the chocolate chips down slightly into the surface so they adhere. This will create a delightful chocolate layer once chilled.

Chill and Set: Place the baking dish into the refrigerator for at least 2 hours, or until the bars are completely firm and the chocolate has set. You'll know they're ready when the entire slab feels solid to the touch and the chocolate has lost its glossy sheen, looking opaque and firm.

Slice and Serve: Once fully chilled, use the foil overhangs to lift the large slab of bars out of the dish and onto a cutting board. Using a sharp knife, cut into 12-16 evenly sized bars. For cleaner cuts, you can warm your knife slightly under hot water and wipe it dry between cuts. Serve immediately or store for later enjoyment.