Preheat your oven to 350°F (175°C). Grease and lightly flour a 9-inch bundt pan or a 9×5-inch loaf pan. (Tip: use non-stick spray and dust with flour for easy release.)

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, cream the softened butter and 1 1/2 cups sugar until light and fluffy, about 2–3 minutes with a hand mixer or stand mixer on medium speed. Scrape down the sides.

Add eggs, one at a time, beating briefly after each until incorporated. Stir in the vanilla extract.

Alternate adding the dry ingredients and the buttermilk into the butter mixture, beginning and ending with the dry (about 3 additions dry, 2 additions wet). Mix until just combined — don’t overmix.

In a small bowl, whisk the cocoa powder with hot water (or coffee) until smooth. Stir in the optional 1/4 cup sugar if you like a sweeter chocolate ribbon.

Fold about 1 1/2 cups of the plain batter into the chocolate mixture to make the chocolate ribbon batter.

Spoon half of the plain (vanilla) batter into the prepared pan. Dollop spoonfuls of the chocolate batter over it, then top with the remaining vanilla batter. Use a skewer or butter knife to gently swirl through the batter a few times to create marbled ribbons — don’t overdo the swirls or the cake will be fully mixed.

Bake for 50–60 minutes (bundt) or 45–55 minutes (loaf), until the top is golden and a toothpick inserted into the center comes out clean or with a few moist crumbs. Rotate the pan halfway through baking if your oven runs hot.

Let the cake cool in the pan for 10–15 minutes, then invert onto a wire rack to cool completely. If you want, dust with powdered sugar or drizzle with a simple melted chocolate glaze: melt 1/2 cup dark chocolate chips with 2 tbsp cream, stir until glossy, and pour over cooled cake.