In a medium saucepan, combine 1 cup of water, 1 cup of granulated sugar, and ½ cup of dark cocoa powder. Heat over medium heat until it boils, then cool completely.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1 ½ cups of granulated sugar, ¾ cup of cocoa powder, 1 ½ teaspoons of baking powder, 1 ½ teaspoons of baking soda, and 1 teaspoon of fine salt.

In another bowl, mix 1 cup of full-fat sour cream, ½ cup of vegetable oil, 3 large eggs, and 2 teaspoons of vanilla extract until smooth. Gradually combine the wet and dry ingredients until just mixed.

Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans. Divide the batter evenly and bake for 23-26 minutes.

Once baked, let the cakes cool in the pans for 10 minutes before transferring to a wire rack.

While the cakes cool, prepare frosting by beating 1 cup of unsalted butter until creamy. Gradually mix in ½ cup of dark cocoa powder and 4 cups of powdered sugar, alternating with ¼ cup of heavy whipping cream.

Level the cake layers if necessary. Soak one cake layer with chocolate syrup, spread chocolate buttercream, and add the second layer on top.

Frost the top and sides of the cake. Chill for about 10 minutes before finishing decorations.