In a medium pot, place butter & mini marshmallows. Melt on medium heat stirring constantly. When mixture is smooth, add vanilla. Turn off heat & add cereal, combine well. Place about 3/4 of the mixture into the prepared pan. GENTLY press into the baking pan. If you pack them down too hard the bars will end up hard & crunchy opposed to soft & gooey. Sprinkle the pepita seeds over the sticky rice krispies.