5 tablespoons (71 grams) unsalted butter 1 1/4 cups (249 grams) granulated sugar 2 large eggs plus 1 egg yolk, cold 1 teaspoon vanilla extract 1/3 cup vegetable oil 3/4 cup (75 grams) unsweetened cocoa powder 1/2 cup (63 grams) all-purpose flour 1/8 teaspoon baking soda 1 tablespoon cornstarch 1/4 teaspoon salt 3/4 cup (128 grams) semisweet chocolate chips