First, make the puree. Add raspberries, sugar, and water to a medium saucepan and heat to a boil. Boil for 5 minutes until mixture is slightly thicker than water and raspberries are broken down. Strain mixture to remove all seeds.

Rinse and clean saucepan (or use a new saucepan) and add milk. Heat to medium until milk is steaming, but not boiling. Add cocoa and powder sugar. Whisk into milk. Add puree and whisk until combined.

Remove from heat and split into two mugs. Add marshmallows and serve immediately.