Line a mini muffin tin with paper liners (this small size is available where cake decorating supplies are sold).
In a microwave safe bowl, melt the butter & peanut butter together for 45 seconds. Stir until smooth.
Add graham cracker crumbs and sugar and mix to combine.
Distribute mixture evenly between the prepared muffin cups.
Refrigerate for 15 minutes to set peanut butter layer.
After the peanut butter has chilled, combine chocolate and milk in a microwave safe bowl and heat in 25 second intervals until chocolate is just melted. Stir until smooth.
Distribute chocolate evenly over the top of the peanut butter layer.
Chill until firm, at least 30 minutes.
Store covered in the fridge.