Preheat oven to 350F (175C) and line a 13x9 baking dish with parchment paper². Set aside.
In a large bowl, combine melted butter, cocoa powder, and sugars and stir until well-combined.
Add eggs/egg yolk and vanilla extract and stir well.
In a separate bowl, whisk together flour, cornstarch, and salt.
Stir flour mixture into wet ingredients until completely combined.
Evenly spread batter into prepared baking dish and transfer to 350F (175C) preheated oven. Bake 18 minutes or until a toothpick inserted in the center comes out with a few moist crumbs or clean. Allow brownies to cool completely in their pan before proceeding.
ROCKY ROAD TOPPING
Once brownies have cooled, prepare rocky road topping by combining chocolate chips, heavy cream, corn syrup and salt in a medium-sized saucepan over low/medium-low heat.
Stir frequently until chocolate chips are completely melted and mixture is smooth.
Remove from heat and allow to cool for ten minutes before stirring in mini marshmallows and chopped almonds. Stir just until marshmallows/almonds are coated in chocolate and then immediately pour over brownies. Use a spatula to spread if needed so the topping is even. Top with a sprinkling of sea salt, if desired.
Allow brownies to sit and the chocolate to set (this could take several hours, or you can speed up the process by setting the brownies in the fridge) before slicing and serving. Enjoy!
¹You may omit the corn syrup if you prefer. I like to use it because it makes the ganache topping shinier and helps it to set a bit more firmly.²Alternatively you can lightly grease and flour the pan or spray with baking spray. I like to use parchment paper because once the brownies cool they can easily be lifted out of the baking pan and cut.