In a medium bowl whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt. Add in eggs, milk, oil, and vanilla extract and using an electric mixer, beat for 2 minutes. Slowly stir in boiling water then pour into prepared pan and cook until a clean toothpick inserted comes out clean, about 40 minutes. Top with marshmallows and place back in oven for one minute more. Remove from oven and cool.