Garnish: chocolate curls (see Kitchen Tip), chopped toasted pecans
Chocolate Frosting
1½ cups unsalted butter, softened
½ teaspoon kosher salt
4 cups confectioners’ sugar, divided
2 tablespoons whole milk, divided
¾ cup chopped semisweet chocolate, melted and slightly cooled
Instructions
Preheat oven to 350°. Line a 13×9-inch baking pan with parchment paper, letting excess extend over sides of pan.
In a large bowl, whisk together mayonnaise, milk, and vanilla. In a medium bowl, whisk together flour, sugar, cocoa, and baking soda. Gradually add flour mixture to mayonnaise mixture, stirring until well combined. Pour batter into prepared pan, smoothing top.
Bake until a wooden pick inserted in center comes out clean, 28 to 30 minutes. Let cool completely in pan on a wire rack. Using excess parchment as handles, remove from pan. Spread Chocolate Frosting onto cooled cake. Garnish with chocolate shavings and pecans, if desired.
Chocolate Frosting
In a large bowl, beat butter and salt with a mixer at medium speed until creamy, about 1 minute. Gradually add 2 cups confectioners’ sugar and 1 tablespoon milk, beating until smooth, about 1 minute total. Gradually add remaining 2 cups confectioners’ sugar and remaining 1 tablespoon milk, beating until smooth and fluffy. Add melted chocolate, and beat until well combined and very fluffy, 3 to 5 minutes. Use immediately.