In a large mixing bowl, add the peanut butter, protein powder, honey, and vanilla extract. Using a spatula or spoon, mix everything together slowly at first, then more firmly until the mixture becomes smooth, thick, and fully combined. Make sure there are no dry pockets of protein powder left. If the mixture feels too thick or hard to mix, add 1–2 tablespoons of milk and continue mixing until it reaches a soft, workable consistency.
Add the crispy rice cereal to the mixture and gently fold it in using a spatula. Mix carefully so the cereal stays crunchy and doesn’t get crushed. Continue folding until all the cereal is evenly coated with the peanut butter mixture, ensuring every bite will have that perfect crunch.
Line a baking tray or dish with parchment paper, making sure the paper covers the sides for easy removal later. Transfer the mixture into the tray and spread it out evenly. Use the back of a spoon or spatula to press it down firmly into a compact, flat layer. Take your time with this step, as pressing it well ensures the bars hold their shape and don’t crumble after setting.
Place the dark chocolate in a microwave-safe bowl or a small saucepan. If using a microwave, heat in 20–30 second intervals, stirring in between each interval until fully melted and smooth. If using a stovetop, melt it gently over low heat while stirring continuously to prevent burning.
Pour the melted chocolate over the pressed base and spread it evenly using the back of a spoon or a spatula. Make sure the entire surface is covered in a smooth, even layer for a clean, glossy finish once set.
Place the tray in the refrigerator and let it chill for at least 1–2 hours, or until the bars are completely firm. The chocolate should harden and the base should feel solid when pressed lightly.
Once fully set, remove the tray from the refrigerator and lift the bars out using the parchment paper. Place on a cutting board and use a sharp knife to slice into evenly sized bars or squares. Serve immediately or store in an airtight container for later.