Brownie Base
1-Preheat the oven to 350°F: Preheat the oven to 350°F. Line a 9×9 baking pan with parchment, leaving an overhang for easy removal.
2-In a medium size microwave safe bowl,: 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.
3-In a large mixing bowl, add the: In a large mixing bowl, add the granulated sugar.
4-Whisk in the chocolate mixture until fully: Whisk in the chocolate mixture until fully combined. (The mixture will be thick.)
5-Whisk in eggs one at a time: Whisk in eggs one at a time until just incorporated. Do not overbeat, you don’t want to add extra air.
6-Add the vanilla extract: Add the vanilla extract.
7-Sift flour, cocoa powder, baking powder and: Sift flour, cocoa powder, baking powder and salt over the wet mixture. Gently fold until no dry streaks remain.
8-Spread batter evenly in pan: Spread batter evenly in pan. Bake for 22 to 25 minutes, or until the edges are set but the center still looks slightly underbaked.
9-Let brownies cool completely in the pan: Let brownies cool completely in the pan while you make the chocolate mousse.

Chocolate Mousse
10-Place the bowl of a stand mixer: 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.
11-In a medium saucepan over low heat,: In a medium saucepan over low heat, combine the chopped chocolate, mini marshmallows, and milk.
12-Stir constantly until the marshmallows and chocolate: Stir constantly until the marshmallows and chocolate are fully melted and smooth.
13-Let cool to room temperature: Let cool to room temperature. This step is important so the heat doesn’t deflate the whipped cream.
14-In a large mixing bowl, beat the: In a large mixing bowl, beat the heavy cream with an electric mixer until soft peaks form.
15-Lower the mixer speed to low, and: 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.
16-Using either a silicone spatula, or an: 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.
17-Remove the brownies from the refrigerator and: 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.