Preheat oven to 350°F
Butter and flour three 8" diameter cake pans with 1 1/2-inch-high sides, or two 9" pans.
In a medium bowl, combine the chopped chocolate and cocoa powder, then whisk in boiling water until chocolate is melted and smooth.
Sift flour, baking soda, and baking powder into another medium bowl. Set aside.
Using mixer, beat both sugars and mayonnaise in large bowl until well blended, 2 to 3 minutes. Add eggs 1 at a time, beating until well blended after each addition. Beat in vanilla.
Add flour mixture alternately with chocolate mixture, beating until blended after each addition.
Bake cakes until tester inserted into center comes out clean, 25-30 minutes.
Cool cakes in pans on racks 20 minutes.
Run small knife around sides of cakes to loosen.
Carefully invert cakes onto racks and let cool completely.
Frost when completely cooled.