• Combine coffee and cocoa:

  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.

  Prepare cake pans by brushing vegetable oil on all surfaces. Dust with flour, dumping out excess.
• Make cake batter:

  Whisk together flour, baking soda, baking powder, sugar and ½ teaspoon of salt in a bowl. Beat in mayonnaise and 1 ½ teaspoons of the vanilla on low speed. Alternately add flour mixture and coffee 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.
• Melt chocolate chips:

  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, the remaining ¼ teaspoon salt, 2 cups of powdered sugar, and the remaining ½ teaspoon vanilla in a bowl of a heavy-duty stand mixer; beat on low speed until smooth. Beat the chocolate mixture at low speed until smooth. Beat in the remaining 2 cups powdered sugar and, if needed, the remaining 1 tablespoon of cream, 1 teaspoon at a time, until a spreadable consistency is reached.

Assemble cake:

• Place 1 cake layer on a serving plate. Spread ½ cup frosting over top. Top with second layer; spread ½ cup frosting over top. Top with third layer. Spread remaining frosting over sides and top of cake.