In large bowl stir together cereal, raisins, peanuts and sunflower seeds until combined; set aside. In heavy 5-quart pot, combine peanut butter and butter.

Place over low to medium-low heat and stir until melted and well blended. Add marshmallows and salt; stir until melted and well blended. Remove from heat and stir in vanilla. Immediately add cereal mixture and quickly stir until cereal is well coated.

Using lightly buttered spatula, quickly spread mixture evenly and firmly in greased 13x9"-inch baking pan. Let stand in pan on rack until completey cool. Cut into 2x2"-inch squares.

Make ahead: Cover pan with foil and store at room temperature for up to 3 days.