In a large bowl, combine the all-purpose flour, granulated sugar, cornstarch, baking powder, cocoa powder, and salt.
In a small bowl, combine the eggs, 1 ½ cups of heavy whipping cream, the vanilla extract, and 5 tablespoons of butter, whisking to combine.
Add the wet ingredients to the dry ingredients and mix until combined.
Pour the batter into a sheet pan prepared with parchment paper and greased with butter. Jiggle the pan slightly to spread the batter evenly across the pan.
Heat the remaining ½ cup of heavy whipping cream (either in the microwave or on the stove top), until it simmers.
Pour the hot heavy cream over the dark chocolate chips in a bowl, and allow to sit for a minute or two before stirring to combine.
Using a spoon, drop dollops of the chocolate sauce over the entire sheet pancake.
Heat the mini marshmallows in the microwave for about 45 seconds, stirring until smooth.
Drop dollops of the melted marshmallow over the sheet pancake.
Using a butter knife, slowly drag it through the batter to swirl the chocolate and marshmallow through the batter.
Bake for 10-15 minutes, checking the middle with a toothpick to confirm doneness.
If desired, top with remaining chocolate sauce and additional marshmallows.