Preheat your oven to 350°F (177°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the long sides to lift the bars out later. Lightly spray the parchment and any exposed sides of the pan with nonstick spray.
nonstick spray, parchment paper
In a large microwave-safe bowl, melt the butter in 20–30 second bursts until just melted. Let it cool for about 5 minutes, until warm but not hot.
1 cup unsalted butter
Add the granulated sugar and brown sugar to the warm butter and whisk until smooth and glossy. Add the eggs, egg yolk, and vanilla, whisking well after each addition until the mixture is thick and shiny.
1 3/4 cups granulated sugar, 1/4 cup light brown sugar, 3 large eggs, 1 large egg yolk, 2 teaspoons pure vanilla extract
Whisk in the cocoa powder, red food coloring, vinegar, and salt until fully combined and evenly colored. If the batter looks too pale, add a bit more food coloring to taste.
2 tablespoons unsweetened cocoa powder, 1 tablespoon red food coloring, 1 teaspoon distilled white vinegar, 1/2 teaspoon fine sea salt
Sprinkle the flour over the wet mixture and gently fold it in with a spatula just until no dry streaks remain. Fold in the mini chocolate chips, being careful not to overmix.
1 1/3 cups all-purpose flour, 1/2 cup mini semi-sweet chocolate chips
Pour the thick batter into the prepared pan. Smooth the top with an offset spatula or the back of a spoon, nudging it into the corners. If desired, sprinkle extra chocolate chips over the top.
1/2 cup mini semi-sweet chocolate chips
Bake on the center rack for 24–30 minutes, starting to check around 22–23 minutes. The edges should look set and start to pull slightly from the sides, while the center should be puffed and just barely glossy. A toothpick inserted about 2 inches from the edge should come out with a few moist crumbs, not wet batter.
Set the pan on a wire rack and let the brownies cool completely in the pan, about 1 hour. Do not cut while warm, as the texture finishes setting as they cool.
In a medium bowl, beat the softened cream cheese and softened butter together until smooth and creamy. Add the powdered sugar and vanilla and beat until thick and fluffy. If the frosting is too stiff, add milk or cream 1 teaspoon at a time until it is easily spreadable.
4 ounces cream cheese, 2 tablespoons unsalted butter, 1 cup powdered sugar, 1/2 teaspoon pure vanilla extract, 1 tablespoon milk or heavy cream
Once the bars are completely cool, spread the cream cheese frosting over the top in an even layer. For the neatest slices, chill the frosted pan in the refrigerator for 30–45 minutes before cutting.
4 ounces cream cheese, 2 tablespoons unsalted butter, 1 cup powdered sugar, 1/2 teaspoon pure vanilla extract, 1 tablespoon milk or heavy cream
Use the parchment overhang to lift the slab out of the pan and transfer to a cutting board. Cut into 16 large or up to 24 smaller bars, wiping the knife clean between cuts for sharp edges.