Make the Syrup:

1. Combine water and granulated sugar in a small saucepan.
2. Heat until the sugar dissolves, then whisk in dark cocoa powder.
3. Set aside to cool to room temperature.

2. Prepare the Cake Layers:

1. Preheat oven to 350°F (175°C). Grease and line three 8-inch round pans with parchment paper.
2. In a large bowl, whisk together flour, granulated sugar, dark cocoa powder, baking powder, baking soda, and salt.
3. In another bowl, mix warm water, sour cream, vegetable oil, eggs, vanilla extract, and instant espresso (if using).
4. Gradually add wet ingredients to dry ingredients, mixing until just combined.
5. Divide batter evenly among prepared pans.
6. Bake for 23-26 minutes or until a toothpick inserted in the center comes out clean.
7. Cool in pans for 15 minutes, then transfer to wire racks to cool completely.
8. Trim tops of cakes to level the layers.

3. Make the Buttercream Frosting:

1. Beat unsalted butter in a large mixing bowl until smooth and creamy.
2. Add dark cocoa powder, vanilla extract, and salt, mixing well.
3. Gradually add powdered sugar, alternating with heavy cream, until frosting reaches desired consistency.
4. Mix in melted dark chocolate chips for a rich, silky finish.

4. Assemble the Cake:

1. Brush each cake layer with cooled syrup to enhance moisture and chocolate flavor.
2. Spread a generous layer of buttercream between each cake layer.
3. Cover the entire cake with buttercream, smoothing with an offset spatula.

5. Decorate the Cake:

1. Press mini chocolate chips around the sides of the cake for added texture.
2. Pipe decorative swirls or rosettes on top using remaining buttercream.
3. Garnish with additional chocolate chips or shavings for a stunning presentation.