Place milk, cocoa powder and sugar in a small saucepan. Heat over medium/medium-low heat, whisking frequently, until warm (but not boiling). Add chocolate chips and whisk constantly until the chocolate chips melt and dissolve into the milk. Whisk in vanilla extract.
Toast the croissant. Then spread with butter on the inside of each half.
Serve the croissant with hot chocolate, and add some mini marshmallows to the hot chocolate if using.