Whisk together the yogurt, eggs, sugar, vanilla, and citrus zest until they look glossy and slightly thickened.
In another bowl, mix the flour, baking powder, and salt.
Stir the dry ingredients into the wet ones, just until the last streaks of flour disappear.
Stream in the oil and melted butter, watching the batter loosen and shine.
Let the bowl sit for ten minutes while the oven preheats to 350°F (175°C) and prepare a loaf pan or round tin lined with parchment.
Pour the batter into the prepared pan and bake until golden and a toothpick comes out with a few moist crumbs, about 40–50 minutes.
