Preheat the oven to 350°F. Line a 9x9 baking pan with parchment, leaving an overhang for easy removal.

In a medium size microwave safe bowl, melt the chopped semisweet chocolate and butter (microwave in 20 second intervals or over a double boiler). Stir in the vegetable oil until smooth.

In a large mixing bowl, add the granulated sugar.

Whisk in the chocolate mixture until fully combined. (The mixture will be thick.)

Whisk in eggs one at a time until just incorporated. Do not overbeat, you don’t want to add extra air. Add the vanilla extract.

Sift flour, cocoa powder, baking powder and salt over the wet mixture. Gently fold until no dry streaks remain.

Spread batter evenly in pan. Bake for 22 to 25 minutes, or until the edges are set but the center still looks slightly underbaked.

Let brownies cool completely in the pan while you make the chocolate mousse.

Place the bowl of a stand mixer and wire whip attachment, or a medium size mixing bowl and the tines of a handheld mixer in the freezer.

In a medium saucepan over low heat, combine the chopped chocolate, mini marshmallows, and milk.

Stir constantly until the marshmallows and chocolate are fully melted and smooth.

Let cool to room temperature. This step is important so the heat doesn’t deflate the whipped cream.

In a large mixing bowl, beat the heavy cream with an electric mixer until soft peaks form.

Lower the mixer speed to low, and add the cooled chocolate mixture. Increase the mixer speed to high and mix until fully combined and stiff peaks form — do not overwhip.

Using either a silicone spatula, or an offset spatula, spread the chocolate mousse over the brownies. Cover and chill in the refrigerator for 2 hours.

Remove the brownies from the refrigerator and slice into 12 equal squares. Garnish with chocolate syrup drizzle if desired. Keep refrigerated until ready to serve.