In a blender, put milk, cocoa powder and sugar. Blend and taste for desired sweetness and chocolateyness. (Did I just make up a new word?)
Add canned pineapple chunks, with some of the juice, and blend. While blender is on, add frozen peaches (or any frozen fruit you like). Blend until thick.
Will drinking the milkshake while reading the mystery help me figure out who did it or how it was done? Only time will tell.