Prepare Chocolate Simple Syrup by combining equal parts of water, granulated sugar, and dark cocoa powder in a small saucepan. Stir over medium heat until it boils and remove from heat to cool.

Preheat oven to 350°F (175°C) and prepare cake pans for baking by greasing and lining with parchment paper.

Sift together the all-purpose flour, dark cocoa powder, baking powder, baking soda, and fine salt in a mixing bowl. In a separate bowl, whisk warm water, sour cream, vegetable oil, eggs, and vanilla. Combine wet and dry ingredients until just mixed.

Evenly divide the batter between the prepared pans. Bake for 22-25 minutes or until a toothpick inserted in the center comes out clean.

Once baked, allow the cake layers to cool in the pans for about 15 minutes, then transfer them to wire racks to cool completely.

Beat softened unsalted butter in a mixing bowl until smooth. Gradually add dark cocoa powder and powdered sugar, alternating with heavy whipping cream, until frosting reaches a spreadable consistency.

Fold in melted dark chocolate chips after allowing them to cool slightly for an extra chocolate kick.

Brush each cooled cake layer generously with chocolate simple syrup. Place one layer on a cake stand, spread buttercream frosting, and sprinkle with mini chocolate chips. Repeat with the second layer.

Coat the outside of the cake with a thicker layer of buttercream frosting, smoothing it out for a polished look. Press mini chocolate chips into the sides of the frosted cake.

Chill in the freezer for about 10 minutes before serving.