Preheat oven to 375°F/190C. Lightly brush the inside of each ramekin with a thin layer of butter. Pour about a tablespoon of sugar into the greased ramekin and move the ramekin around to get a thin layer of sugar on the inside. Pour the sugar in the other ramekin and repeat. Dump out any excess sugar and set aside. In a small bowl, whisk together egg yolk, mango puree, yogurt, salt and cardamom until smooth. Add cake flour and mix until well combined. In a separate bowl, add egg white and cream of tartar and whisk on high. When the whites are frothy, slowly add the granulated sugar into the mixture and whip until you have soft peaks. Add 1/3 of the whipped egg whites to the yogurt mixture and mix until well combined. Fold in the rest of the egg whites gently until there are now streaks in the batter. Spoon the batter into the ramekins and run your thumb along the edge of the ramekin to clean it up. Place the ramekins onto a baking sheet and bake for 15 - 16 minutes. Dust the tops with powdered sugar and enjoy with a drizzle of mango puree!