In a saucepan on medium heat add 1 cup of raspberry puree, a white sugar. Stir to combine ingredients and bring to a simmer.
Whisk together remaining raspberry puree, and cornstarch until smooth. Add to heating raspberry/sugar mixture, and stir to combine. Cook for five to six minutes until thickened and glossy. Strain through a fine mesh sieve and allow to completely cool to room temperature.
In a stand mixer (or with a hand mixer) add egg whites and cream of tartar and whisk on medium speed until foamy in texture (approximately one minute).
Increase speed, and slowly stream in sugar. Continue whisking until egg whites have formed stiffed peaks and have tripled in volume.
Add 1/3 of whipped egg whites to raspberry mixture, and mix well to incorporate. Then add another 1/3 of the egg whites, this time gently folding so as to contain as much of the air in the egg whites as possible. Then add the final 1/3 of whipped egg whites, and gently fold until no white streaks can be seen.
Grease ramekins (one cup capacity) with butter, and dust interior with white sugar so that all sides are coated evenly.
Fill ramekins to the top with soufflé mixture, and level off top with an offset spatula. Place your thumb towards the edge of the ramekin, and turn the ramekin to remove a little souffle batter to create a tiny canal. This will help the souffle rise.
Place on a baking sheet and bake for eight to ten minutes at 350 degrees or until the volume of the souffle has increased by a 1/3 of the size of the ramekin.
Remove from oven and garnish with a sifting of powdered sugar and a dollop of fresh whipped cream. Enjoy! :)