Preheat your oven to 350 degrees Fahrenheit. While it’s heating up, line your 12-cup cupcake tin with cupcake liners. A little tip: lightly spray the liners with non-stick spray for easy removal later.
In a microwave-safe bowl, melt the canola oil, butter, and chocolate chips together. Heat them in 30-second increments, stirring in between. You can also achieve this over low heat on the stove, but the microwave is my favorite method for ease. Once melted, whisk the mixture until it’s completely smooth and set it aside to cool.
In a medium-sized bowl, combine flour, baking soda, baking powder, cocoa powder, and salt. Stir everything together until well mixed, then set it aside.
In a large mixing bowl, whisk together the eggs, egg yolk, granulated sugar, and vanilla extract. Beat the mixture until it’s smooth and creamy.
Add the cooled butter, oil, and chocolate mixture to the egg mixture. Whisk until everything is well combined. Next, add half of the flour mixture and half of the sour cream. Stir gently to combine, then repeat with the remaining flour and sour cream. Be careful not to over-mix; this is key for fluffy cupcakes!
Quickly stir in the hot coffee. The heat will enhance the chocolate flavor, so don’t skip this step! Gently fold the mixture until everything looks uniform.
Divide the batter evenly among the lined cupcake tins, filling each liner about two-thirds full. Bake in the preheated oven for 16 to 18 minutes. When they’re done, a toothpick inserted in the center should come out clean. Let them cool completely in the pan.
While the cupcakes cool, sift together the confectioners sugar and cocoa powder for the frosting. This helps to avoid lumps.
In a mixing bowl, use a handheld mixer or a stand mixer fitted with a paddle attachment to beat the softened butter on medium-high speed for about 2 minutes until it’s creamy. Reduce the speed to low and gradually add the sifted sugar and cocoa powder, alternating with the sour cream and milk. Once everything is added, increase speed to medium-high and beat until the frosting is fluffy, about another 2 minutes.
Once the cupcakes are completely cool, frost them as desired. You can go simple or create beautiful swirls; the choice is yours! For an extra touch, drizzle some chocolate sauce on top and sprinkle with holiday sprinkles and a toasted marshmallow.
To make the chocolate drizzle, place the chopped bittersweet chocolate in a bowl. In a small saucepan, warm the heavy cream over medium heat until it's hot but not boiling. Pour the cream over the chocolate and let it sit for 2 to 3 minutes. Whisk until smooth, then let it cool slightly before drizzling over the frosted cupcakes.
To toast the marshmallows, place them on a baking sheet and position your oven rack right below the broiler. Preheat the broiler, then toast the marshmallows until golden brown, watching closely as they can burn quickly. Once toasted, let them cool for a minute before placing one on each cupcake.