Preheat your oven to 350°F (175°C). Grease and line three 9-inch round cake pans with parchment paper to ensure cakes do not stick.

In a large bowl, whisk together the all-purpose flour, white sugar, light brown sugar, dark cocoa powder, baking soda, and salt until evenly combined.

Stir in the cooking oil and melted unsalted butter into the dry ingredients until the mixture looks crumbly and combined.

Beat the large eggs and vanilla extract into the batter, mixing thoroughly to combine everything evenly.

Slowly pour in the buttermilk while mixing gently to maintain a smooth batter without overmixing.

Add the hot water or coffee gradually and mix until the batter is smooth and thin. The hot liquid helps to bloom the cocoa for a deeper chocolate flavor.

Evenly distribute the batter among the prepared pans. Bake in the preheated oven for 22–25 minutes or until a toothpick inserted in the center comes out clean.

Let the cakes cool in their pans for about 10 minutes, then remove them and transfer onto wire racks to cool completely before frosting.

Beat the softened butter until creamy. Gradually add the cocoa powder, powdered sugar, heavy cream, vanilla extract, and pinch of salt. Continue beating until smooth and fluffy.

Place the first cooled cake layer on a serving plate. Spread a generous amount of frosting on top, then repeat with remaining layers. Use the remaining frosting to cover the top and sides of the cake. Decorate as desired.

Store the cake at room temperature covered lightly for 1–2 days. For longer storage, keep the frosted cake in an airtight container in the refrigerator for up to 5 days.

Wrap unfrosted cake layers tightly in plastic wrap and aluminum foil and freeze for up to 2 months. Thaw completely at room temperature before frosting and serving.