Step 1: Cut both ends off the oranges. One at a time, using a sharp paring knife, remove the peel and all the white pith from each orange in strips, working from top to bottom all the way around the orange. Cut the oranges crosswise in half and place them on a plate cut side up. Step 2: Sprinkle the rosemary leaves over the oranges, and push some of the rosemary leaves into them so they adhere. Sprinkle half the sugar on top. Step 3: Heat a chapa or large cast-iron skillet over medium-high heat. Spread the remaining sugar on the cooking surface and when it begins to melt, put the oranges cut side down on the sugar. Do not move them for 3 to 4 minutes, and adjust the heat so that the cut side burns nicely but does not smell acrid and the oranges are softened. Step 4: Transfer “burnt” side up to individual plates. Spoon the yogurt next to the oranges. Drizzle the burnt sugar and juices from the pan over the oranges and yogurt, and serve.