Get 2 slices of bread and add the chocolate and top with marshmallows, press the other slice of bread over it. Reserve.
In a bowl big enough to immerse the bread slices. Add the warm milk, eggs, cocoa, and vanilla extract then whisk it well.
Dip the toasts quickly into the mixture and reserve on a plate.
In a frying pan, melt the butter and add the french toasts. Keep at medium-low heat to avoid burning the butter and the toast, once the marshmallow is melted, remove it from the heat. In conclusion, Serve immediately.