Add the chocolate, peanut butter, maple syrup, protein powder, and milk to a small saucepan. Heat over low heat, stirring frequently, until everything is melted and smooth.
Remove the saucepan from the heat and stir in the vanilla extract until well combined.
Transfer the chocolate mixture to a very large mixing bowl. Add the crispy rice cereal and gently fold everything together until the cereal is evenly coated. Try not to crush the cereal too much; you want to keep that crispy texture.
Line a pan with parchment paper, leaving a little overhang for easy removal. Transfer the mixture to the pan and spread it out evenly. Press firmly. This step is important! Use the back of a spoon, spatula, or another piece of parchment paper to really press the mixture down firmly. The more compact it is, the better the bars will hold together. Place the pan in the freezer for 2-3 hours, or until the bars are firm enough to slice.
Remove from the pan, cut into bars, and sprinkle with a little sea salt if you like.
Store the bars in an airtight container in the freezer for up to 3 months. They’re perfect to grab straight from the freezer whenever you’re craving something chocolatey and crispy.