Add cocoa powder, sugar, baking soda, and coffee powder in a mixing bowl. Add boiling water and vanilla essence to this mixture. Stir well and prepare a smooth mixture. Now keep it aside.
Preheat oven to 180 C and prepare a cake tin, grease the sides, and base and line it with parchment paper.
Sift the dry ingredients 2-3 times.
Cream butter and sugar until light and fluffy. Add in eggs, one at a time and mix it until incorporated well.
Add vanilla essence and mix it well.
Now add dry ingredients to the wet ingredients in 2-3 batches. Mix it with a spatula using the cut & fold method or use a beater on low speed.
Add in milk and mix till the cake batter is smooth and homogenous. Do not over-mix it.
Divide the cake batter into 3 equal parts. Add 4 tablespoons of the cocoa powder mixture in one part and mix it well.
Now add the remaining cocoa powder mixture to the second part of the cake batter and mix it well.
Pour the dark cocoa layer into the greased pan. Gently level it with a spoon and tap it twice on the counter to remove air bubbles.
Gently pour the second chocolate layer into the greased pan. Gently spread it with a spoon and tap the pan twice to release the air bubbles.
Now, pour the remaining batter into the pan. Level it with a spoon and tap it twice.
Bake the cake for 35-40 minutes or until the skewer inserted in the middle comes out clean.
Remove from the oven and let it cool down for 5 minutes. Remove from the cake pan and let it cool down on the wire rack.
After cooling down completely, slice the cake to the desired thickness.
Chocolate ombre cake is ready to serve.