In a small saucepan, melt butter; stir in crumbs. Reserving 1 T for garnish, pat crumbs on bottom and sides of a 9-inch pie plate. Chill while preparing the filling.
In a large bowl, beat cream cheese until light and fluffy; add peanut butter and sweetened condensed milk, beating until smooth. Stir in lemon juice (if using) and vanilla, fold in whipped cream.
Pour into crust and garnish with reserved crumbs. Chill 4 hours (or freeze). Refrigerate leftovers.