1. Preheat oven to 350°F. Spread soft butter into 4-5 ramekins. Sprinkle evenly with 2-3 tablespoons granulated sugar. Place ramekins in a roasting pan and set aside.
2. Defrost blueberries in a microwave oven or at room temperature. Place in a blender and puree until super smooth.
3. Press mixture through a sieve and measure 1/3 cup blueberry puree/juice.
4. In a bowl whisk together 1/3 cup blueberry puree, buttermilk, lemon zest, and sugar. Whisk in cornstarch, salt, and egg yolks.
5. In a separate bowl whip egg whites to medium peaks. Fold gently into above mixture and until all streaks of blueberry puree are gone.
6. Fill ramekins 7/8 full.
7. Pour water in roasting pan until it reaches halfway to the sides of ramekins.
8. Place in oven and after 10 minutes reduce heat to 325°F, bake for a total of about 40-45 minutes. The cakes will have risen, and edges will be golden to light brown. Remove from oven.
9. Meanwhile prepare blueberry sauce.
10. Defrost blueberries and puree until super smooth. Running mixture through a sieve is optional. Add sugar. If desired add just a touch of vodka, orange liquor and or lemon juice to brighten flavor.
11. Once pudding cakes are removed from oven let rest for 3-4 minutes and turn ramekins upside down onto plates.
12. Surround with blueberry sauce and mandarin orange segments.