For the Marshmallow Brownie Layer: 

1. Preheat & Prep: Start by preheating the oven to 350°F. Prepare a parchment lined 8×8 inch square pan with baking spray or a homemade pan release. Let the parchment hang over the edges—instant handles
2. Melt Butter: In a microwave-safe bowl, microwave the butter at 15-30 second intervals until completely melted. Set aside.

3. Combine the Ingredients: In a large mixing bowl, add the brownie mix, egg, egg yolk, melted butter, and water. Stir until the batter is almost combined—don’t over-mix!

4. Chocolate Chips: Add the chocolate chips to the brownie batter and gently fold them in with a spatula until evenly distributed.

5. Bake: Bake for 40-45 minutes, or until a toothpick inserted into the center comes out mostly clean with just a few moist crumbs. We don’t want a completely clean toothpick, we want these fudgy!

6. Marshmallow Layer: Turn off the oven and pull the brownies out. Spread the mini marshmallows evenly over the top, then place the pan back in the oven for 5–7 minutes until the marshmallows are soft and puffed – not browned
7. Cool Down: Don’t skip the cooling time! These brownies finish setting as they cool and must be completely cooled before adding the brownie frosting.

For the Brownie Frosting: 

1. Whip the Butter: Start by mixing the butter on medium-high for 30 seconds-1 minute, in a stand mixer or with an electric hand mixer

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2. Vanilla & Salt: Add the vanilla extract and salt.
3. Powdered Sugar: Add the powdered sugar and cocoa powder and start mixing. 
4. Start Heavy Cream: Add heavy cream and continue mixing until incorporated. Scrape the bowl
5. Last Step: Mix on low-medium until all ingredients are incorporated, this could take a few minutes. At this point you should have perfectly smooth, chocolate frosting for your brownies!
6. Spread Over Brownies: Spread the chocolate frosting evenly over the brownies and allow them to cool completely before slicing.