In a large mixing bowl, stir peanut butter, sweetener and vanilla extract together until combined, then add cereal and protein powder and stir again until all rice crispies are coated.
Line a baking sheet or plate with parchment paper and pour mixture onto the paper.
Press down on mixture with a spoon, then mold the outer edges with the spoon so it makes a giant rectangle shape.
Place in refrigerator for 30 minutes to harden.
Once the mixture has hardened, remove from fridge and slice into four rectangles.