Preheat oven to 350°F with rack set in middle. Grease a 9"x5" loaf pan with butter then line it with parchment paper.

Mix together cake flour, baking powder, baking soda, and salt in a medium bowl. Set aside.

Mix together dutch-processed cocoa powder and hot water in a medium bowl. Set aside.

Whip butter for about two minutes until smooth. Add sugar and beat on medium-high for 4 minutes until light and fluffy. Lower speed and add eggs one at a time, mixing well between additions. Add vanilla and mix well.

Alternately add flour mixture (3 parts) and buttermilk (2 parts) in this order: flour - buttermilk - flour - buttermilk - flour. Fully incorporate between each addition but be careful to not overmix.

Transfer 1/3 of the batter into the bowl with the cocoa/water mixture. Stir until just fully mixed together, without overmixing.

Spread the remaining 2/3rds of the vanilla batter over the bottom of a large bowl. Spoon the chocolate batter into several large dollops over the top of the vanilla batter. Using a rubber spatula, carefully fold the batter over itself 2-3 times, moving the batter from the bottom of the bowl onto the top. Don't overdo it or you will lose the marbled effect.

Gently transfer batter into prepared loaf pan then smooth down the top. Smack the bottom of the pan on countertop a couple times to help release any trapped air.

Bake the cake until an inserted toothpick comes out almost clean, about 40-45 minutes. Don't overbake. Cool the cake in the pan for 10 minutes before transferring to a wire rack to cool the rest of the way.

Cool cake completely before decorating. Dust with icing sugar or frost with my fresh cherry frosting. I used a Wilton 4B tip on a piping bag to decorate this cake.