Combine sifted flour mixture (1 ¾ cups self-rising flour, ¼ teaspoon salt, and 1 teaspoon baking soda) with sifted ¾ cup cocoa powder, 1 cup sugar, 1 cup brown sugar,1 ½ teaspoons ground cinnamon, and ⅛ to ¼ teaspoon ground cayenne (depending how spicy you want it) in a large mixing bowl.
Add 2 eggs, ½ cup vegetable oil, 1 cup milk, and 1 ½ teaspoons vanilla extract and beat at medium speed for 2 minutes, until smooth and no lumps remain.
Heat 1 cup prepared coffee (or water) until it's about to boil and add to the chocolate batter in a thin stream, beating at low speed until completely incorporated and smooth.
Pour thin batter into the prepared pan. It will be very thin.
Bake for about 40 minutes, until the top springs and a cake tester or toothpick inserted in the center comes out clean.
Let cake cool completely on a wire rack. Make sure the sides of the cake are not stuck to the pan and remove it carefully, helping lift it with the parchment paper.
Transfer to the serving plate after removing the paper.
Spread cinnamon frosting on top (I use an offset spatula or the back of a spoon) and sprinkle with chopped chocolate (or mini chocolate chips)and ground cinnamon if you want to.
Cut into squares and serve.
Store leftovers in the refrigerator, well covered.