• Preheat the oven to 350°F (180°C). Lightly grease a 9×5-inch (23X13 cm) loaf pan and line it with parchment paper, leaving some overhang for easy removal.
• In a small bowl, whisk together the cocoa powder and hot water until smooth and paste-like. Set aside to cool while you make the batter.
  ¼ cup unsweetened cocoa powder, 3 tablespoons hot water
• In a medium bowl, whisk the flour, baking powder, baking soda, and salt. Set aside.
  1¾ cups all-purpose flour, ½ tablespoon baking powder, ½ teaspoon baking soda, ¼ teaspoon salt
• In a large bowl, beat the butter and sugar with a hand mixer (or stand mixer) until light and fluffy, about 3–4 minutes.
  ¾ cup unsalted butter at room temperature, 1 cup granulated sugar
• Add the eggs one at a time, mixing well after each addition. Scrape down the bowl as needed, then mix in the vanilla.
  2 large eggs at room temperature, 2½ teaspoons pure vanilla extract
• Add half the flour mixture, then half the buttermilk, mixing on low speed until combined. Repeat with the remaining dry ingredients and buttermilk. Mix just until combined, but do not overmix.
  ⅔ cup buttermilk at room temperature
• Transfer about ¾ cup of the vanilla batter to a small bowl. It doesn't have to be exact.

Assemble the cake

• Add the cooled cocoa mixture to the reserved vanilla batter and mix until smooth and fully combined.
• Spread a thin, even layer of vanilla batter over the bottom of the prepared loaf pan.
• Using a regular tablespoon, drop small dollops of chocolate batter (about half a tablespoon each) over the vanilla layer, spacing them out in a loose checkerboard pattern. Smaller chocolate dollops help create a more balanced, well-defined marbled pattern.
• Spoon larger dollops of vanilla batter (about 1 tablespoon) over and between the chocolate dollops, gently covering them without spreading or mixing.
• Continue alternating small chocolate dollops and larger vanilla spoonfuls until all the batter is used. Gently shake the pan to help the batter settle evenly.

Swirl options (choose one):

• No swirl: Leave the batter as is. The cake will naturally form a marbled pattern as it bakes.
• One or two swirls: For more defined ribbons, use the back of a wooden spoon to make 1–2 slow zigzags through the batter from one end of the pan to the other, moving gently left and right. Don’t over-swirl, or the batters will blend together instead of forming a marbled pattern.
• Bake for 60–70 minutes, or until a toothpick inserted into the center comes out clean. After the first 30 minutes, top loosely with foil to prevent browning.
• When the cake is done, let it cool in the pan for 15-20 minutes, then lift it out and transfer it to a wire rack to cool completely
• Slice and enjoy!