1 c. peanut butter chips
1/3 c. milk
1 1/2 c. miniature marshmallows
1 c. heavy cream
1/2 tsp. vanilla

Place peanut butter chips, milk and marshmallows in top of double boiler over simmering water and stir until marshmallows and chips are completely melted. Cool to lukewarm. Whip heavy cream until stiff; fold in vanilla and peanut butter mixture. Use to fill cake.