Make the cake. Preheat oven to 325 degrees. Grease and flour a 13x9" baking pan and set aside.
In a large bowl, sift together the flour, sugar, cocoa, and baking soda.
Add mayonnaise, coffee, and vanilla and stir until combined.
Pour cake batter into prepared pan and bake for 20-25 minutes, or until toothpick inserted in the center comes out clean. Remove from oven and allow to cool completely before frosting.
Make the frosting. Mix first 5 ingredients in top of a double boiler.*
Beat 4-7 minutes until mixture stands in peaks. Remove from heat, exchange hot water at bottom for cold water, and put double boiler together again.
Add vanilla and corn syrup. Beat until peaks hold. Spread over chocolate cake. Enjoy!
Notes
*If, like me, you don't have a double boiler, all you need is a saucepan and a Pyrex glass bowl. You'll be placing the Pyrex bowl on top of the saucepan. First, fill the saucepan with water so that there is about an inch between the bottom of the bowl and the surface of the water. Bring the water to a boil and proceed with recipe as directed.
Recipe by Simple, Sweet & Savory at https://www.simplesweetsavory.com/hundred-dollar-chocolate-cake/