Preheat broiler to high. Spread the broccoli on a large rimmed baking sheet. Drizzle with oil and toss to coat. Space the florets evenly on the pan, then flatten them with the bottom of a mason jar or sturdy glass. Combine za'atar, garlic powder, salt and pepper in a small bowl; sprinkle over the florets. Broil until hot and starting to brown, 4 to 5 minutes. Top each floret with a small dollop of yogurt before serving.