Mix the flour, cornstarch, baking soda, baking powder, salt, cocoa, and coffee in a bowl.
Mix the buttermilk with the vinegar and vanilla.
In another bowl, cream the butter and oil with both types of sugar until frothy and light in color.
Gradually add the eggs, mixing well after each one.
Incorporate the dry ingredients, alternating with the wet ones, mixing on low speed.
Spread in a 30x40cm tray.
Bake at 175°C for 25-30 minutes.
Cool completely, then cut 3 discs of 18cm diameter (the third might be made from 2-3 pieces).
Save the scraps for decoration.