Lightly grease 4 square frames measuring 20cm/8in on each side by 5cm/2in high with nonstick oil spray and then coat them with flour. Line 2 sheet pans with nonstick rubber mats. Place 2 frames in each sheet pan.
Roll the chocolate shortbread out into a 1.25cm/.5in thick tube with your hands and wrap it around the outside of the frame at the base where it comes in contact with the non-stick rubber mat. Flatten it with your fingertips against the frame and the rubber mat. This is to prevent the batter, which is very loose, from seeping out of the frame.
Sift the flour, cocoa powder, salt, baking soda, and baking powder together and place in the bowl of an electric mixer.
Combine the eggs, buttermilk, and coffee in another bowl and whisk together to obtain a homogeneous mix.
Place the bowl with the dry ingredients on a mixer with the paddle attachment on low speed and slowly pour in the egg mixture.
Once the liquid has been completely incorporated, pour in the butter and mix until it has combined completely.
Divide the batter into the prepared frames (about 1.6kg/4lb2.4oz per frame). Bake until the cake springs back when it yields to gentle pressure, 25 to 35 minutes. Or perform a knife test by introducing the tip of a knife down the center of the cake; if it comes out dry, it is done baking.
Let the cakes cool at room temperature. Wrap tightly. If refrigerated, it will keep in peak condition for 3 to 4 days. If frozen, no more than 1 month is recommended.