In a microwave-safe bowl, mix together the peanut butter, corn syrup, and brown sugar. Microwave for about 3 minutes on HIGH. Stir in the ground flaxseed and vanilla, then pour the mixture over Rice Krispies and mix well. Coat a 9x13 inch baking pan with nonstick cooking spray, then spread the mixture into the pan and press down to form a dense sheet. Let the mixture sit for about 5 to 10 minutes, until firmly set, then cut into bars. Makes about 28 bars. Each bar contains approximately 2/3 tbsp. flaxseed.