4 oz. (125 g) bittersweet chocolate, coarsely chopped
2 cups (10 oz./315 g) unbleached all-purpose flour
2 tsp. baking powder
1/4 tsp. fine sea salt
1 cup (8 oz./250 g) granulated sugar
3 large eggs
1/2 tsp. pure vanilla extract
3/4 cup (6 oz./185 g) full-fat Greek-style yogurt
Finely grated zest of 1 lemon, plus 2 Tbs. fresh lemon juice
3/4 cup (6 fl. oz./180 ml) extra-virgin olive oil, plus more for greasing
Preheat an oven to 350°F (180°C).
Put the chocolate in the top of a double boiler and set over (but not touching) barely simmering water. Heat, stirring, until the chocolate has melted. Remove from the heat and let cool slightly. Alternatively, melt chocolate in microwave and let cool.
In a bowl, whisk together the flour, baking powder and salt. Set aside. In a large bowl, using an electric mixer set on high speed, beat together the granulated sugar and eggs until pale in color. Beat in the vanilla. Add half of the flour mixture, beating until just incorporated. Beat in the yogurt and lemon juice, and then the remaining flour mixture. With the motor running, beat in the olive oil just until smooth. Scoop about one-third of the batter into a clean bowl and gently fold in the chocolate. Stir the lemon zest into the rest of the batter.
Lightly oil a 9-by-5-inch (23-by-13-cm) loaf pan. Fill the pan with alternating spoonfuls of the lemon and chocolate batters. Swirl a large, thin-bladed knife in an S pattern through the batters to create the marbled effect. Bake until a cake tester inserted into the center comes out clean, 60 to 65 minutes. Transfer to a wire rack and let cool before slicing.
Adapted from williams-sonoma.com