Lightly oil and flour a 9 inch square baking pan. In a small bowl whisk together the flour, baking powder and salt.
In a medium saucepan melt the bittersweet chocolate with the oil over low heat. Remove from heat and scrape into a medium bowl. Whisk in ¾ cup sugar and 2 teaspoons vanilla. Whisk in the whole eggs, one at a time. Whisk the dry ingredients into the chocolate mixture until the batter is smooth.
In a bowl, beat the mascarpone with the egg yolk, ¼ cup sugar and ½ teaspoon vanilla until smooth.
Pour half of the brownie batter into the baking pan. Dollop half of the mascarpone on top, sprinkle with Baci, if using. Top with remaining batter, dollop on the remaining mascarpone mixture and using a knife or chopstick, swirl the batter a few times to create a marbled effect (do not over swirl or you will simply be mixing the mascarpone into the brownie mixture, you want them to remain separate).
Bake in the center of the oven for 50 to 55 minutes, or until a toothpick inserted in the center comes out with just a few crumbs clinging to it. Let brownies cool before cutting. Store any uneaten brownies covered in the refrigerator.