Preheat oven to 355 degrees F (180 degrees C).
In a mixer or mixing bowl, cream Unsalted Butter (200 gram) and Granulated Sugar (180 gram) until light and fluffy.
Crack in the Egg (3), one at a time.
Drizzle in Vanilla Extract (1 teaspoon).
Gently stir in All-Purpose Flour (170 gram) and Baking Powder (3/4 teaspoon).
In a small bowl, mix Unsweetened Cocoa Powder (3 tablespoon), Activated Black Charcoal (1 1/2 teaspoon), and a couple of splashes of hot water until you have a paste while things are mixing.
Once your batter is well combined, portion 1/3 of the batter into another mixing bowl. Mix in your chocolate paste with the batter thoroughly.
Transfer half of the vanilla batter into the loaf pan. Then, transfer all of the chocolate batter for the second layer. Cover it with the rest of the vanilla batter as the third layer.
Using a fork, swirl the batter, go back and forth for a few times to make sure that the two batter are well-marbled.
Bake it in the oven for about 1 hour and 15 minutes, until a toothpick inserted into the center comes out clean.
Let cool completely before slicing it up. It can keep well for up to 5 days in room temperature.