In a medium heavy pot, combine milk, peanut butter, and desired amount of honey over low heat. Stir just until all ingredients are well combined and warmed through. Add protein powder and oats. Stir to combine well. (If mixture is too thick to stir, simply add a bit more milk.)Lightly grease an 8x8 pan. Press mixture evenly into pan, using enough pressure to compress mixture into desired bar thickness.Allow bars to cool completely.Cut into squares. If desired, drizzle with melted chocolate and let cool. Wrap in airtight container and store at moderate room temp for up to a week.