Preheat the oven to 325°F / 165°C. Generously grease an 8 by 4-inch / 20 by 10cm loaf pan, then line it with greased parchment paper.

In a medium bowl, whisk together both flours, salt, and cream of tartar, then set aside.

In a stand mixer fitted with the paddle attachment, cream the butter on medium-high speed until creamy and smooth, about 1 minute.

Turn the mixer speed to low, add the sugar to the butter, and mix until incorporated. Turn the speed to medium-high and beat until very light and fluffy, about 5 minutes. Scrape the bowl often for even incorporation.

Add the vanilla extract, turn the speed to medium, and mix until incorporated.

Turn the speed to medium-low and add the eggs, one at a time, beating just until combined. Scrape the bowl after each addition.

Add one-third of the flour mixture to the butter mixture, mixing on low speed, just until combined. Add half the reserved drained cherry juice and mix on low speed just until combined. Repeat with another one-third flour, the remaining cherry juice, and the final one-third flour, scraping the bowl and paddle after each addition.

Divide the batter in half. Stir the cocoa powder into half of the batter. Toss 1 Tbsp flour with the drained, thawed cherries. Fold the cherries into the non-cocoa batter.

Using an ice-cream scoop or a spoon, alternate scoops of the vanilla and chocolate batters into the prepared pan. Drag a knife or skewer through the batter tocreate a marbled effect. Smooth the top and tap thepan on the counter a few times to release excess airbubbles. Set the pan on a baking sheet.

Bake until a tester comes out clean, about 1 hour 10 minutes. Let the cake cool in the pan for 10 minutes, then remove from the pan and place onto a serving plate and let cool completely before serving.