Stir together the flour, hot cocoa mix, sugar, baking powder and salt together in a 12 oz mug.
Stir in the milk, vegetable oil and vanilla. Mix well to make sure that all the ingredients are well combined.
Gently stir in the chocolate chips if using.
Microwave on high for 60-90 seconds (until the cake is just set but the center is still a little jiggly). Check the cake after 90 seconds by inserting a toothpick into the center of the cake (not a melted chocolate chip) comes out clean. If the mug cake is not done, cook for 10-15 second intervals until done.
Top with the mini marshmallows when serving and enjoy!
Notes
With mug cakes, you want to make sure that you do not overcook the mug cake. Stop cooking the cake earlier than you think as the mug cake will continue cooking as it sets.