The original recipe called for whole Greek yogurt, but I had low-fat on hand so that’s what I used here. If you have trouble finding barley flour, substitute all-purpose instead. Next time I bake this cake, I want to drop in a handful of blueberries or a pinch of chopped herbs (rosemary would be nice) from the garden.
Preheat the oven to 350 F. Butter a standard (8 1/4 by 4 1/4 inch) loaf pan and dust with flour; tap out excess.
Whisk the flours, baking powder and salt together in a medium bowl. Using your fingers, rub the sugar with the lemon zest in a large bowl until the sugar becomes moist and fragrant. Add the yogurt, oil, eggs and vanilla and whisk until combined.
Fold in dry ingredients just to blend. Pour batter into prepared pan and smooth out the top. Bake for 40-45 minutes or until fragrant and the top of the cake is golden brown and a tester inserted into the center comes out clean. Cool for 15 minutes before serving. Store covered at room temperature for up to 3 days.