How to Make It
Whisk together hot coffee and cocoa in a bowl. Let stand until room temperature, about 20 minutes. Preheat oven to 350°F. Grease and flour 3 (9-inch) round cake pans.
Whisk together flour, baking soda, baking powder, and 1⁄2 teaspoon of the salt in a bowl. Combine granulated sugar and eggs in a large bowl. Beat with an electric mixer on medium-low speed until light and fluffy, about 4 minutes. Beat in mayonnaise and 1 1⁄2 teaspoons of the vanilla on low speed. Alternately add flour mixture and coffee mixture to egg mixture in 5 additions, beginning and ending with flour mixture. Divide batter evenly among prepared pans. Bake in preheated oven until a wooden pick inserted in center comes out with moist crumbs, 22 to 25 minutes.
Cool cake layers in pans on a wire rack 20 minutes. Remove cake layers from pans; cool completely on rack, about 30 minutes.
Place chocolate chips, light corn syrup, and 3 tablespoons of the heavy cream in a microwavable bowl. Microwave on HIGH until smooth, about 1 minute, stirring every 15 seconds. Let stand until room temperature, 10 minutes.
Combine butter, remaining 1⁄4 teaspoon salt, 2 cups of the powdered sugar, and remaining 1⁄2 teaspoon vanilla in bowl of a heavy-duty stand mixer; beat on low speed until smooth. Beat in chocolate mixture on low speed until smooth. Beat in remaining 2 cups powdered sugar and, if needed, remaining 1 tablespoon cream, 1 teaspoon at a time until spreadable consistency is reached.
Place 1 cake layer on a serving plate. Spread 1⁄2 cup frosting over top. Top with second layer; spread 1⁄2 cup frosting over top. Top with third layer. Spread remaining frosting over sides and top of cake.