Preheat oven to 400°F (200°C). Line a baking sheet with parchment. Spread the halved, pitted cherries in a single layer and lightly sprinkle with sugar. Roast for 10 minutes until glossy and slightly jammy; set aside to cool.

Reduce oven temperature to 325°F (163°C). Spray an 8x8-inch pan with nonstick spray, line the bottom and two sides with parchment, and spray again.

In a bowl, sift together the flour, Dutch-process cocoa powder, cornstarch, and salt; set aside.

In a stand mixer fitted with a whisk (or with a hand mixer), beat the granulated sugar, eggs, and egg yolk on high speed for 5 minutes until pale, thick, and voluminous.

With the mixer on low, stream in the melted (cooled) butter, oil, and vanilla until just combined.

Fold the dry ingredients into the egg mixture with a rubber spatula in two additions, mixing gently until no dry streaks remain.

Fold in half of the roasted cherries and all of the chocolate chips.

Scrape the batter into the prepared pan and smooth the top. Bake at 325°F (163°C) for 40–50 minutes, until the edges are set and a toothpick inserted toward the center comes out with a few moist crumbs (not wet batter).

Cool completely on a wire rack. For clean cuts, chill the pan in the freezer for 15 minutes.

Lift out using the parchment, scatter the remaining roasted cherries on top, press lightly to adhere, slice, and serve. Optional: sprinkle extra chocolate chips on the hot brownies right after baking so they soften on top.