Preheat the oven to 350 F/ 180 C degrees. Grease an 8 ½ x 4 ¼ x 2 ½-inch loaf pan (2lb loaf tin). Line the bottom with parchment paper. Grease and flour the pan. Sift together the flour, cocoa, baking powder, and salt into one bowl. In another bowl, whisk together the yogurt, 1 cup (200g) sugar, the eggs, orange zest, and vanilla. Slowly whisk the dry ingredients into the wet ingredients. With a rubber spatula, fold the vegetable oil into the batter, making sure it’s all incorporated. Pour the batter into the prepared pan and bake for about 50 minutes, or until a cake tester placed in the center of the loaf comes out clean.