Preheat your oven to 350 degrees F. This temperature is perfect for baking the cake, ensuring it rises beautifully. While the oven heats up, prepare a 13X18 rimmed baking sheet by lining it with parchment paper. Don’t forget to spray the parchment with nonstick cooking spray so the cake comes out easily!
In a large mixing bowl, add granulated sugar, all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt. Use an electric mixer to mix the dry ingredients well. This step is crucial for evenly distributing the leavening agents and ensuring a consistent cake texture.
In a separate bowl or measuring pitcher, combine the egg, buttermilk, vegetable oil, and vanilla extract. Whisk these wet ingredients until well combined. This mixture will add moisture and flavor to the cake.
Slowly pour the wet mixture into the dry mixture while the mixer is on low. Mix until everything is incorporated. Turn the mixer to medium speed and beat for about 1 minute. This will help to aerate the batter, making your cake light and fluffy.
Use a rubber spatula to scrape down the sides of the bowl to ensure all ingredients are mixed well. Turn the mixer back on low and beat in the hot coffee until the batter is smooth. The addition of hot coffee enhances the chocolate flavor, creating a rich taste.
Pour the cake batter into the prepared baking pan. Gently shake the pan back and forth to even out the batter. Bake in the preheated oven for 11 to 13 minutes, or until the cake is soft yet fluffy in the center. Check for doneness by inserting a toothpick; it should come out clean.
Once baked, allow the cake to cool for about 15 minutes at room temperature. After that, transfer the cake to the freezer to cool completely until hard, which takes about 1 to 2 hours. Chilling the cake will make it easier to cut and assemble.
Once the cake is firm, carefully cut it in half with a serrated knife. Take one half and spread a generous layer of Nutella across its surface. This creamy hazelnut spread will add moisture and flavor to the finished product.
Take the other half of the cake and place it on top of the Nutella-covered half. Use the parchment paper to help lift it if necessary. Once layered, trim any rough edges with a serrated knife, then cut the cake into 1 inch squares. You should have around 100 petit fours.
Place the baking sheet with the petit fours back into the freezer and chill until they are rock-hard, which takes another 1 to 2 hours. This step is crucial for the next part!
Meanwhile, melt the dark chocolate chips with vegetable shortening in a small saucepan over low heat. Stir gently until the chocolate is smooth, or you can microwave it in one-minute increments, stirring in between each time until melted.
Prepare a large rimmed baking sheet with paper muffin liners. Press 1 or 2 liners flat at a time. Take only 1 to 2 mini cakes out of the freezer at a time to keep them cold. Insert a wooden skewer into the center of a cake and quickly dunk it into the melted chocolate. Use a spatula to splash chocolate over the top if needed.
Once coated, tap the skewer on the edge of the pot to remove excess chocolate. This helps keep the coating thin. Quickly transfer the petit four to the muffin liner and twist the skewer to remove it. Fill the tiny hole left behind with melted chocolate using a skewer or small spoon. Sprinkle decorative candy sprinkles over the chocolate before it hardens.
Repeat this process, working quickly to keep the majority of the cakes frozen and reheating the chocolate as needed, until all petit fours are coated.
Once finished, cover the baking sheet with plastic wrap to keep them fresh until serving!
