Preheat oven to 325°F. Grease a 13x9 inch baking dish.
Place the cake mix, water, mayonnaise, and eggs in a large bowl. Beat with an electric mixer for 3 minutes. Pour into the prepared pan.
Bake for 30–32 minutes, or until a toothpick comes out clean. Let cool completely.
For the frosting, place the unsweetened chocolate in a microwave-safe bowl. Microwave in 20 second increments until melted.
Place the softened butter, melted chocolate, powdered sugar, vanilla, and 2 tablespoons milk in a large bowl. Use a handheld mixer to mix until it reaches frosting-like consistency and the powdered sugar is incorporated. Add an additional tablespoon of milk if the mixture looks too stiff.
Spread the frosting over the cooled cake. Cut into slices and serve.
Notes
Use full-fat (regular) mayonnaise for this cake. Low fat mayo will not create the same result.The cake can be stored at room temperature for 3–4 days or in the refrigerator for up to 1 week. Cover the cake tightly before storing.You can freeze the cake in a tightly covered container for up to 2 months. Defrost at room temperature overnight before serving.