- 1/2 cup (1 stick) unsalted butter, room temperature, plus more for pan
- 6 ounces blackberries (1 1/3 cups)
- 1 1/4 cups plus 2 tablespoons sugar
- 1 1/2 cups all-purpose flour
- 1/2 teaspoon coarse salt
- 1/4 teaspoon baking powder
- 2 large eggs, lightly beaten
- 1/2 teaspoon pure vanilla extract
- 1/2 cup sour cream, room temperature
1. Preheat oven to 350 degrees. Lightly butter a 5-by-9-inch loaf pan and line with parchment, leaving a 2-inch overhang on all sides; butter parchment. In a food processor, puree blackberries with 2 tablespoons sugar. In a medium bowl, whisk together flour, salt, and baking powder.
2. In a large bowl, using an electric mixer, beat together butter and 1 1/4 cups sugar until light and fluffy, 5 minutes. Add eggs and vanilla and beat to combine, scraping down bowl as needed. With mixer on low, add flour mixture in 3 additions, alternating with sour cream, beginning and ending with flour mixture.
3. Transfer half the batter to pan and dot with 1/2 cup blackberry puree. Repeat with remaining batter and puree. With a skewer or thin-bladed knife, swirl batter and puree together. Bake until golden brown and a toothpick inserted in center of cake comes out clean, about 1 1/4 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 slicing.