½ cup (1 stick - 4 oz) unsalted butter, room temperature, plus more for pan (or ½ cup neutral oil, like canola or coconut, OR ¼ cup (4 tablespoons) butter plus ¼ cup oil)
3 ounces blackberries (about a scant ¾ cup)
3 ounces raspberries (about a scant ¾ cup)
1¼ cups plus 2 tablespoons sugar
1½ cups all-purpose flour
½ teaspoon coarse salt
¼ teaspoon baking powder
2 large eggs, lightly beaten
½ teaspoon pure vanilla extract
½ cup Greek Yogurt, room temperature
directions:
Preheat oven to 350 degrees F. Lightly butter a 9-by-5-inch loaf pan and line with parchment paper, leaving a 2-inch overhang on all sides; then butter parchment. In a food processor, puree blackberries with 1 tablespoon sugar. Wipe out processor and puree raspberries with 1 tablespoon of sugar. Pour/scrape into separate bowls and set aside (you can strain them into the bowls if you don't like the light bite of seeds that do not break down). In a medium bowl, whisk together flour, salt, and baking powder.
In a large bowl, using an electric mixer, beat together butter (or oil, or a combo of the two as mentioned in ingredient list) and 1¼ cups sugar until light and fluffy, about 5 minutes. Add eggs and vanilla and beat to combine, scraping down bowl as you go. With mixer on low speed, add flour mixture in 3 additions, alternating with Greek yogurt, beginning and ending with flour mixture.
Stir two to three tablespoons of the cake batter into the bowl with the blackberry puree until uniform. Stir two to three tablespoons of the cake batter into the bowl of raspberry puree, until uniform.
Pour half the plain batter into the pan and dot with ½ of the blackberry puree -batter and half the raspberry puree-batter. It will seem like it takes over all the plain cake batter, but don't worry, it all works out in the end. Swirl/marble lightly using a skewer or knife. Top with remaining plain batter and dot with remaning raspberry and blackberry batter as you did with the first layer. Again, swirl the puree-batter mixes into the plain batter - pushing a skewer or knife all the way to the bottom for a full marble.
Bake until golden brown and a toothpick inserted in center of cake comes out clean, about 1¼ hours. Let cool in pan on a wire rack, 30 minutes. Lift cake out of pan and place on a serving plate; let cool completely before serving.
Recipe by parsley sage sweet at https://parsleysagesweet.com/2011/08/08/berries-yogurt-flour-eggs-butter-vanilla-and-sugar/