Preheat your oven to 350°F (175°C). Line an 8×8-inch pan with parchment paper or lightly grease it.
In a heatproof bowl, melt the chopped semisweet chocolate and butter together over a saucepan of simmering water (or in 20-second microwave intervals). Stir until smooth.
Stir in vegetable oil and sugar. Let cool slightly, then whisk in the eggs and vanilla extract until smooth and glossy.
In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt. Fold the dry ingredients into the wet mixture just until combined—do not overmix.
Spread the brownie batter evenly in the prepared pan and bake for 25–30 minutes, or until a toothpick comes out with a few moist crumbs. Cool completely before topping.
2️⃣ Add the Marshmallow Layer
While the brownies are still warm, sprinkle 1 cup of mini marshmallows evenly over the top.
Return to the oven for 2–3 minutes until the marshmallows begin to puff and soften slightly. Let cool completely before adding mousse.
3️⃣ Prepare the Chocolate Mousse
In a small saucepan, warm the whole milk over medium heat until steaming (do not boil). Pour over chopped chocolate (if part of mousse—assume ~4 oz semisweet chocolate) and let sit 2 minutes. Stir until smooth and let cool slightly.
In a chilled mixing bowl, beat the cold heavy cream until soft peaks form.
Fold the cooled chocolate mixture gently into the whipped cream in batches until no streaks remain. Chill mousse if needed for 10–15 minutes to thicken before spreading.
Spread mousse evenly over the cooled marshmallow brownie base. Use an offset spatula for a smooth finish.
Refrigerate for at least 2 hours before slicing for neat layers.
