Fudgy Brownie Slices

1. Preheat oven to 325 °F.
2. Add the sugars and eggs to the bowl of a stand mixer fitted with the paddle attachment; cream on high speed until light and fluffy.
3. Meanwhile, melt the butter by microwaving in a heatproof bowl/container at 30-second increments, stirring between each, then add the chocolate and stir until it's fully melted and combined with butter.
4. Turn down the speed of the mixer to the lowest setting and add the butter/chocolate mixture, oil, and vanilla.
5. Sift together flour, cocoa, cornstarch, and salt, and add to the wet ingredients while the mixer is still on low speed. Mix just until combined.
6. Fold the 1/2 cup of chocolate with a silicone spatula.
7. Grease a 10-inch round silicone mold or springform with nonstick spray, pour in the brownie batter, and bake on the middle rack for 35-45 minutes (until an inserted toothpick comes out mostly clean).
8. Cool to room temperature before removing from the mold/form, cut into 8 slices, and decorate as desired.