Cake – Preheat oven to 350F. Line a 9-by-9-inch square baking pan with aluminum foil, spray with cooking spray, or grease and flour the pan; set aside. (Don’t use a 9-inch round pan, it’s too small.)
To a large mixing bowl add the egg, sugar, sour cream, oil, peppermint extract (be careful, measure, and don’t over-add; peppermint extract is very potent), and whisk until smooth and combined.
Add 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, optional salt, and whisk vigorously until batter has 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 32 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.
Allow cake to cool completely in pan on top of a wire rack, at least 30 minutes, before adding ganache.
Chocolate Ganache – To a medium microwave-safe bowl, add the chocolate chips and heat on high power for 1 minute to soften (they won’t melt); set aside.
In a small microwave-safe bowl or glass measuring cup, heat the cream (I used half-and-half) on high power just until it begins to bubble and show signs of boiling, about 60 to 75 seconds.
Pour cream over chocolate chips and let it stand about 1 minute.
Whisk vigorously until chocolate has melted and mixture is smooth and combined.
Evenly pour ganache over cake, smoothing lightly with a spatula if necessary.
Allow ganache to set up and cool for about 5 minutes while you chop the peppermint candies. If you add them while ganache is too warm, they’ll melt. Too cool, and they won’t adhere; use your judgment.
Dice and evenly sprinkle the peppermint candies over cake.
Allow cake to cool uncovered at room temp for at least 2 to 3 hours, or until ganache has set up, before slicing and serving. I am comfortable storing this cake airtight at room temp for a day or two, and after that I store airtight in the fridge for up to 1 week. Chilling it has a nice effect with the chocolate and peppermint.