Preheat oven to 350 F. Grease or use parchment paper to line a 9x9-inch baking pan.
In a large saucepan, melt butter.
¾ cup butter
Turn heat to low, and stir in sugar. Let cook, stirring constantly, for 2-3 minutes.
1 ½ cups granulated sugar
Remove from heat, and stir in vanilla.
1 teaspoon vanilla
Let mixture cool for about 5 minutes.
Whisk in eggs until combined.
2 large eggs
Add flour, cocoa powder, and salt. Whisk in until just incorporated.
¾ cup all-purpose flour, ½ cup unsweetened cocoa powder, ½ teaspoon salt
Pour batter into prepared pan, spreading batter as needed.
Bake for 20-30 minutes. The edges should look done, but the center should still look shiny. A toothpick inserted in the center should come out with a few fudgy crumbs.
Cool brownies completely in the pan.
To make the mousse, heat marshmallows, chocolate, and milk in a medium saucepan over low heat, stirring constantly until completely melted and smooth.
¾ cup miniature marshmallows, 4 ounces semi-sweet baking chocolate, ¼ cup milk
Cool completely. It's important that this mixture is completely cool. Otherwise you will end up with liquid mousse.
In a separate bowl, beat whipping cream to stiff peaks.
1 cup heavy whipping cream
Fold in cooled chocolate mixture.
Spread mousse evenly over brownies.
Cover, and chill brownies for at least 30 minutes.
To make the whipped cream, beat cream and sugar together until stiff peaks form.
1 cup heavy whipping cream, ⅓ cup granulated sugar
Spoon whipped cream over the chocolate layer.**
Brownies can be served at this point, or you can chill them for cleaner cut lines.
Cover, and chill leftover brownies.
Video
Notes
Butter - I use salted butter. If you're using unsalted butter, increase the salt to 1 teaspoon.
Unsweetened cocoa powder - I've used Dutch-processed and regular unsweetened cocoa powder. Either one will work.
Miniature marshmallows - I find miniature marshmallows easier to work with, but full-size marshmallows will work.
Semi-sweet baking chocolate - It's important to use baking chocolate so that it will melt nicely.
Milk - Any milk from skim to whole milk will work.
Prep time doesn't include cooling or chilling times.