Gather the ingredients - * Butter * Sugar * Eggs * Vanilla extract * Flour * Cocoa powder * Coffee powder * Salt * Chocolate chips * Mini marshmallows * Milk * Heavy cream
In a large bowl, stir together the butter and sugar before whisking in the eggs and vanilla.
Stir in the flour, cocoa powder, espresso powder (optional), and salt until combined.
Fold in the chocolate chips and then transfer the batter to the prepared pan.
Bake for 25 minutes, or until a toothpick inserted into the center comes out clean. Let full cool.
In a large saucepan over medium-low heat, melt the chocolate chips, marshmallows, and milk until smooth. Allow to cool to room temperature.
In the bowl of a stand mixer fitted with the whisk attachment, whip the cream until stiff peaks form. Gently fold in the chocolate mixture until smooth.
Spread the mousse onto the cooled brownies, then allow to chill in the fridge for at least 4 hours before slicing and serving.