In a small saucepan, combine equal parts granulated sugar and water and bring to a gentle boil over medium heat. Once dissolved, whisk in ¼ cup of dark cocoa powder until smooth. Allow the mixture to simmer for about 2-3 minutes before removing it from heat. Let the chocolate simple syrup cool completely before setting it aside for later use.

Preheat your oven to 350°F (175°C). While it heats, grease three 8-inch or four 7-inch cake pans with butter or cooking spray, ensuring even coverage. Line the bottom of each pan with parchment paper for easy removal later.

In a large mixing bowl, sift together the dry ingredients: all-purpose flour, granulated sugar, dark cocoa powder, baking powder, baking soda, and fine salt. Stir with a whisk until fully combined.

In a separate bowl, carefully whisk together the wet ingredients: warm water, full-fat sour cream, vegetable oil, eggs, and vanilla extract. Add instant espresso if desired. Pour the wet mixture into the dry ingredients and stir gently until just incorporated.

Divide the batter evenly among the prepared pans and bake for about 22-26 minutes or until a toothpick inserted in the center comes out with moist crumbs.

Allow the cakes to cool in the pans for 15 minutes, then invert onto wire racks and peel off the parchment paper. Let the layers cool completely, about 1 hour.

In a medium bowl, beat softened unsalted butter until creamy. Gradually add cocoa powder and powdered sugar, alternating with heavy whipping cream. Mix until light and fluffy, then fold in melted dark chocolate.

Brush the tops of each cooled cake layer with chocolate simple syrup. Generously frost each layer with the buttercream and sprinkle with mini chocolate chips. Stack the layers neatly and freeze for 15 minutes for stability.

Frost the entire outside of your Death by Chocolate Cake with remaining buttercream, coating with mini chocolate chips for decoration. Slice and serve.