Cake – Preheat oven to 350F. Line a 9-by-9-inch square baking pan with aluminum foil (leave overhang) and spray with cooking spray; set aside. (Don’t use a 9-inch round pan, it’s too small.)
To a large mixing bowl add the egg, sugar, yogurt, oil, and whisk until smooth and combined.
Add the coffee, cocoa powder, and whisk vigorously until batter is smooth and free from lumps. Alternatively use a handheld electric mixer to quickly break up cocoa lumps. Note about coffee – The coffee does not make the cake taste like a coffee-flavored cake. Rather, it brings out and enhances the flavor of the cocoa powder and adds depth of flavor; I highly recommend coffee but use water if you must. The coffee can be any temperature other than piping hot; you don’t want to scramble the egg.
Add the flour, baking soda, baking powder, salt, and whisk until just combined, about 1 minute; don’t overmix.
Turn batter out into prepared pan, smoothing the top lightly with a spatula if necessary, and bake for about 25 to 29 minutes, or until top has set and a toothpick or cake tester inserted in the center comes out clean. I’ve found that climate and brands of ingredients used impact the baking time for this cake greatly, and have baked it in as short as 25 minutes and as long as 32; bake until it’s done in your oven. Set cake aside to cool completely in pan on top of a wire rack.
After cake has cooled, using the aluminum foil overhang, lift cake out onto cutting board and using a 3 to 4-inch diameter cookie-cutter that’s been sprayed with cooking spray (helps cake release much cleaner from the cutter), cut 4 circles out; set aside. Enjoy nibbling on the scraps.
Hot Fudge – Make your own hot fudge or use a thick store bought hot fudge. Do not use ice cream ‘sundae sauce’ (i.e. Hershey’s Syrup or similar) because it’s too thin and runny and won’t hold up well.
Tip -I recommend assembling the cake layers very shortly before serving because they may have a tendency to slide around if you assemble too far in advance. Place one cake round on a serving platter and top with about 3 tablespoons hot fudge and spread it evenly. Repeat with the cake/hot fudge layering process until finished. When you place a cake layer on top of the hot fudge, it will naturally run down the sides of the cake. The more hot fudge you add, the greater the run-down factor; this is not a neat and tidy cake.
Optionally add sprinkles to the top of the cake before serving. Cake is best fresh but will keep airtight for up to 5 days at room temp.