Line an 8 x 8-inch baking dish with 2 sheets of parchment paper. One going in each direction. Set aside.
Pour 3 cups of rice cereal in a large bowl and set aside.
In a medium saucepan over medium-low heat, add the peanut butter, maple syrup, and vanilla extract (*see note). Using a heat-safe spatula, stir until softened and completely combined.
Remove from heat and add the cocoa powder and stir gently to combine. It will take 1-2 minutes to fully incorporate.
Pour mixture on top of the rice cereal. Using a spatula, gently stir to combine. Stir until the cereal is evenly coated. The mixture will be thick and may seem like it won't cover the cereal, but just keep stirring, it will be enough.
Pour and spread mixture into prepared baking dish. Press down with a spatula to make it even all around. This helps everything stick together better too.
Place in the freezer for 20 minutes. When finished, pull up by each side of the parchment paper. Place on cutting board and cut into squares. Store in an airtight container at room temp for 4-5 days. You can also freeze them for 1-2 months.