1 Cup (125 grams) frozen blackberries
1 ½ Cup (204 grams) all-purpose flour (plus 2 teaspoons for berries)
2 teaspoons baking powder
¼ teaspoon fine sea salt
1 Cup (200 grams) sugar
Grated zest of 1 lemon or mandarin orange
2 teaspoons finely chopped fresh rosemary
½ Cup (120 ml) plain yogurt, at room temperature
3 large eggs, at room temperature
½ Cup (120 ml) olive oil
Crème fraîche, for serving
Blackberry purée, for serving
Roughly chop the frozen blackberries and place them on a plate lined with a double thickness of paper towels. Cover with another double layer of paper towels and set aside to thaw for 20 minutes.
Center a rack in the oven and preheat it to 350°F. Generously butter an 8 1/2-x-4 1/2-inch loaf pan (or coat it with baker’s spray).
Whisk the flour, baking powder and salt together in a small bowl and set aside.
Put the sugar in a medium bowl, add the grated zest and rosemary and, working with your fingertips, rub the ingredients together until the sugar is moist and aromatic. Now, whisk in the yogurt. One by one, whisk in the eggs, then give the mixture a couple of energetic beats, so that it’s very well blended.
Still working with the whisk, stir in the dry ingredients, whisking just until blended. Switch to a large, flexible spatula and gradually fold in the oil. You’ll have a thick, smooth batter with a light sheen.
Remove the blackberries from the paper towel to the plate. Sprinkle with 2 teaspoons flour and use your fingers to toss the berries until they’re coated. Drop the berries into the batter and use the spatula to gently fold them in. Scrape the batter into the pan, smoothing the top.
Bake for 55 – 65 minutes or – more important than time – until a cake tester inserted deep into the center of the cake comes out clean. (Depending on how cold the berries are, the cake could take longer to bake.) Transfer the pan to a cooling rack, wait 5 minutes, then, if necessary, run a table knife between the cake and the sides of the pan to loosen the cake. Unmold onto the rack and cool the cake to room temperature right-side up.
If you’d like, serve with crème fraîche and blackberry purée. The cake keeps best at room temperature. Wrap it well and it will hold for at least 4 days.