Preheat the oven to 325F/163C convection (300F/149C conventional). Use cooking spray and parchment paper to grease and line an 8-inch square baking pan. Use binder clips to secure the paper to the pan.
In a microwave safe bowl or a saucepan, melt the butter with the milk chocolate and mix until they're smooth. Set this aside to cool while you get the rest of the ingredients together.
3/4 cup (170g) unsalted butter, 6 ounces milk chocolate chips or bar
Use a whisk to combine the granulated sugar, brown sugar, and eggs- don't over-mix.* Combine the flour, cocoa powder and salt in a medium bowl.
1 cup (120g) all-purpose flour, 1/4 cup (25g) Dutch-process cocoa powder, 4 large eggs, 1 cup (200g) granulated sugar, 1/2 cup (95g) light brown sugar, 1/2 tsp fine sea salt
While whisking, slowly pour in the butter and chocolate mixture in to the eggs and sugar. Add the dry ingredients into the wet ingredients and fold everything together just until they're barely combined.
Pour the brownie batter into the prepared pan and bake for about 35-40 minutes. Remove the brownies from the oven and allow them to cool in the pan for about 15-30 minutes.
Increase the oven temperature to 450F (232C).
Combine all the ingredients for the chocolate sauce in a small bowl. Drizzle half of the chocolate on top of the brownies and then top with 8 oz of mini marshmallows.
8 oz mini marshmallows, 1 package of hot cocoa mix, 1-2 tsp water
Place the brownies with the marshmallows back into the oven for 2-5 minutes until the marshmallows are toasted.** Remove them from the oven and allow them to cool for another 10-15 minutes in the pan.
Drizzle the leftover chocolate sauce on top of the marshmallows and slice into 16 squares.