Preheat the oven to 350 degrees F. Set out a 13X18 rimmed baking sheet and line it with parchment paper. Spray the parchment with nonstick cooking spray.
In the bowl of an electric mixer, add the sugar, flour, cocoa powder, baking powder, baking soda, and salt. Mix well.
In a separate bowl (or measuring pitcher) combine the egg, buttermilk, oil, and vanilla extract. Whisk to combine.
Turn the mixer on low and slowly pour the wet mixture into the dry mixture. Once fully incorporated, turn the mixer on medium and beat for 1 minute. Scrape the bowl with a rubber spatula. Then turn on low again and beat in the hot coffee. Once smooth, turn off the mixer.
Pour the cake batter into the prepared pan. Gently shake the pan to even out the batter. Then bake for 11-13 minutes, until soft and fluffy in the middle.
Cool the cake for 15 minutes at room temperature. Then move the cake pan to the freezer and freeze until hard. (about 1-2 hours)
Once the cake is firm, cut it in half. Spread the Nutella in a thin layer over one-half of the cake. Then use the parchment paper to lift the naked half of the cake over the top of the Nutella to create one cake with two layers. Peel off the parchment paper. Use a serrated knife to cut off the rough edges, then cut the cake into 1 inch squares. You should have approximately 100 petit fours. Place the baking sheet back into the freezer until the cakes are very hard. (about 1-2 hours)
Once the cake is rock-hard, place the chocolate chips in a small saucepot or in a microwave-safe bowl. Add the vegetable shortening. Warm over low heat, stirring until melted and smooth. Or microwave in one-minute increments, stirring in between, until smooth.
Lay paper muffin liners on a large rimmed baking sheet. Press 1-2 liners flat at a time. Take only 1-2 mini cakes out of the freezer at a time, so they stay cold and hard, until ready to dunk. Poke a wooden skewer into the center of one cake and quickly dunk it in melted chocolate. (Sometimes it helps to have a spatula to splash the chocolate over the top. Do this fast so the cake doesn’t soften and fall off the skewer.)
Tap the skewer on the edge of the pot, to remove excess chocolate. (You want to keep it thin.) Then quickly move the petit four to one of the flattened paper liners. Twist the skewer to remove it from the cake and quickly fill the hole with melted chocolate, using a skewer or small spoon. Before the chocolate hardens, sprinkle it with decorative candy sprinkles.
Repeat, repeat, repeat, making sure to keep the majority of the mini cakes frozen, and rewarming the chocolate as needed, so it thinly coats the cakes.
Once finished, cover the baking sheet with plastic to keep fresh.
Keep at room temperature for 2-3 days or in the refrigerator for up to a week.