Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper and spread the halved cherries evenly. Sprinkle with a little sugar. Roast for 10 minutes, then let them cool.
Reduce the oven temperature to 325°F (165°C). Spray an 8×8 inch baking pan with non-stick baking spray, and line the bottom and two sides with parchment paper. Spray again for easy release.
In a large mixing bowl, sift together the flour, cocoa powder, cornstarch, and salt. Set aside.
Using an electric mixer fitted with a whisk attachment, beat the sugar, eggs, and egg yolk on high speed for about 5 minutes until pale and fluffy.
Meanwhile, melt the butter and let it cool slightly. Then, on low speed, add the melted butter, oil, and vanilla to the egg mixture. Remove from the mixer.
Gently fold the sifted dry ingredients into the wet ingredients using a rubber spatula. Be careful not to deflate the batter.
Fold in half of the roasted cherries and the chocolate chips until evenly combined. Pour the brownie batter into the prepared pan, spreading it out evenly.
Bake the brownies for 40-50 minutes, or until the edges are set but the center remains slightly gooey. While still warm, top with extra chocolate chips if desired.
Let the brownies cool completely in the pan. Freeze the brownies for 15 minutes to make slicing easier.
Remove from the pan, top with the remaining roasted cherries, slice, and serve.