Make the Chocolate Simple Syrup: In a medium saucepan, combine equal parts water and granulated sugar over medium heat. Stir continuously until boiling, then whisk in dark cocoa powder until fully dissolved. Allow to cool.

Prepare Cake Batter: Preheat oven to 350°F (175°C) and grease two round cake pans. Whisk together all-purpose flour, baking powder, baking soda, fine salt, and dark cocoa powder. Add warm water, sour cream, vegetable oil, eggs, and vanilla extract. Mix until just combined.

Bake: Pour cake batter into prepared pans, filling them about two-thirds full. Bake for 22-25 minutes, or until a toothpick comes out with a few moist crumbs.

Cool Cake Layers: Cool layers in pans for 15 minutes, run a knife around edges, and invert onto wire racks. Let layers cool completely.

Level Layers: Trim the tops of cooled cake layers using a serrated knife for even stacking.

Make Buttercream Frosting: Beat room temperature unsalted butter until creamy. Gradually sift in dark cocoa powder, salt, vanilla extract, and heavy whipping cream. Add powdered sugar until a smooth consistency is reached.

Assemble Layer Cake: Soak the top of each cake layer with cooled chocolate syrup, spread buttercream between layers, and apply a crumb coat before refrigerating.

Chill and Serve: Frost the cake with remaining buttercream, optionally sprinkle mini chocolate chips, chill for easier slicing, and serve within 2 days at room temperature.