In a small saucepan, sprinkle gelatin over cold water; let stand for 1 minute. Heat over low heat, stirring until gelatin is completely dissolved; set aside. In a large saucepan, heat milk to 175°; stir in sugar and salt until dissolved. Whisk a small amount of the hot mixture into the eggs. Return all to the pan, whisking constantly. Cook and stir over low heat until mixture reaches at least 160° and coats the back of a metal spoon. Remove from the heat. Add peanut butter cups and softened gelatin; stir until melted. Cool quickly by placing pan in a bowl of ice water; stir for 2 minutes. Stir in evaporated milk and vanilla. Press waxed paper onto surface of custard. Refrigerate for several hours or overnight. Fill cylinder of ice cream freezer two-thirds full; freeze according to the manufacturer’s directions. Refrigerate remaining mixture until ready to freeze. When ice cream is frozen, transfer to a freezer container; freeze for 2-4 hours before serving.