In sauce pan over medium heat-melt butter and peanut butter together stirring constantly. Once melted take off the heat and beat in sifted powdered sugar 1 cup at a time until it forms a cookie dough like ball. Stir in vanilla.
Pat into a buttered 8x8x2inch pan. Press chocolate chips into peanut butter. Run a butter knife through it to form desired piece size. Cover and place in fridge until cool. Once cool, run a knife around the edge of the pan to loosen. Pop out and serve.
Note: fudge must be cut before cooled or it will break.
Also if you prefer swirled chocolate, melt chips in microwave with 2 tbs. butter- mix and pour over fudge in pan. Swirl butter knife through the peanut butter fudge to create marble effect. Cut into pieces. Cool in fridge.