Preheat oven to 350°. Grease two 9-inch cake pans, line them with rounds of parchment, and grease the parchment; set aside.

In a medium bowl, whisk together the flour, cocoa, baking soda, and baking powder; set aside.

In a large bowl or mixer, beat eggs, sugar, and vanilla on high for 3 minutes or until light and fluffy. Beat in mayonnaise at low speed until blended.

Alternately beat in flour mixture with coffee, beginning and ending with flour mixture. Divide evenly between prepared pans.

Bake for 26-30 minutes or until a toothpick inserted into the center of the cakes comes out clean.

Let cakes cool for about 20 minutes, then remove to cooling racks (remove parchment if it's stuck to the cakes) to cool completely.

Place chopped chocolate into a microwave-safe bowl. Microwave, stopping to stir frequently until the chocolate is melted and smooth.

Let the chocolate cool until lukewarm, stirring occasionally.

Beat butter in a large bowl until smooth and creamy. Sift powdered sugar over the butter and beat until well blended, about 2 minutes. Beat in vanilla.

Add melted chocolate and beat until well blended, scraping down the sides of the bowl as needed.

Frost as desired.