Preheat oven to 350°F. Spray a 9 inch round springform pan or 9 x 9 inch square pan with baking spray. Set aside.
In a large mixing bowl, whisk together flour, granulated sugar, brown sugar, cocoa, baking soda, baking powder, and salt.
Make a well in the center of the mixture and add buttermilk, mayo, and vanilla. Stir by hand to combine or use an electric mixer to beat just until smooth.
Fold in chocolate chips by hand. Pour evenly into prepared pan.
Bake for 45 minutes OR until a toothpick inserted into the center comes back clean.
Cool in the pan on a cooling rack then remove ring and place onto a platter.
To make frosting: Whisk together powdered sugar and cocoa. In separate bowl use an electric mixer to cream butter and vanilla. Lower speed of mixer, adding sugar-cocoa mixture. Beat into butter on low speed, adding hot milk 1 tablespoon at a time repeating until smooth. Frost top of cooled cake.
Store leftovers in an airtight container chilled for up to one week.