Red Velvet Sheet Cake

Ingredients

• Nonstick vegetable oil spray
• 3 oz. unsweetened chocolate, chopped
• ¾ cup sour cream
• 1 Tbsp. plus 1 tsp. liquid red food coloring
• 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt
• 2 tsp. vanilla bean paste or vanilla extract
• 2 cups (250 g) bleached all-purpose flour
• 1¾ cups (packed; 350 g) light brown sugar
• 1 tsp. baking powder
• 1 tsp. baking soda
• ¾ cup (1½ sticks) unsalted butter, cut into 1" pieces, room temperature
• 3 large eggs, lightly beaten to blend
• 8 oz. cream cheese, room temperature
• ½ cup (1 stick) unsalted butter, room temperature
• 2 tsp. vanilla bean paste or vanilla extract
• 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt
• 1 cup (110 g) powdered sugar
• ¼ cup chilled heavy cream
• Flaky sea salt and sprinkles (for serving; optional)

Steps

1. Place a rack in middle of oven; preheat to 350°. Lightly coat bottom and sides of a 13×9" metal baking pan with nonstick vegetable oil spray. Line bottom of pan with parchment paper. Place 3 oz. unsweetened chocolate, chopped, in a medium bowl. Pour 1 cup boiling water over chocolate and whisk until melted and smooth. Add ¾ cup sour cream, 1 Tbsp. plus 1 tsp. liquid red food coloring, 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt, and 2 tsp. vanilla bean paste or vanilla extract; whisk to combine.
2. Beat 2 cups (250 g) bleached all-purpose flour, 1¾ cups (packed; 350 g) light brown sugar, 1 tsp. baking powder, and 1 tsp. baking soda in the bowl of stand mixer fitted with the paddle attachment on low speed until well combined, about 1 minute. Increase speed to medium-low and beat in ¾ cup (1½ sticks) unsalted butter, cut into 1" pieces, room temperature until mixture resembles breadcrumbs, about 1 minute. Scrape in chocolate mixture and beat until batter is smooth and fluffy, about 2 minutes. Beat in 3 large eggs, lightly beaten to blend, in 3 additions, beating until incorporated between additions and scraping down sides of bowl with a rubber spatula as needed; batter should be smooth and creamy. Scrape batter into prepared pan and smooth surface with an offset spatula.
3. Bake cake until set and top springs back when lightly pressed (an instant-read thermometer inserted into the center should register 200°), 35–40 minutes. Transfer to a wire rack and let cake cool in pan, about 1 hour. If you prefer to serve the cake unmolded, invert cooled cake onto rack and carefully peel away parchment, then invert cake, right side up, onto a platter.
4. Beat 8 oz. cream cheese, room temperature, ½ cup (1 stick) unsalted butter, room temperature, 2 tsp. vanilla bean paste or vanilla extract, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in the clean bowl of a stand mixer fitted with the clean paddle attachment on medium-high speed, scraping down sides of bowl halfway through, until light and fluffy, about 4 minutes. Turn off mixer and add 1 cup (110 g) powdered sugar and ¼ cup chilled heavy cream. Starting on low speed and gradually increasing to medium, beat until frosting is smooth and creamy, about 2 minutes.
5. Spread frosting evenly over top of cake, working all the way to the edges with an offset spatula. Using a spoon or offset spatula, create decorative divots and swirls in frosting if desired. Sprinkle with flaky sea salt and sprinkles if desired.