Add frozen mixed berries, water, lemon juice, liquid stevia, and erythritol to a small pot. Heat the ingredients to a simmer, and leave the berries simmering with the pot’s lid on. When the berries turn soft, reduce the heat to bubbling and continue cooking until you can smell some caramelization. No stirring is necessary. Once done, let the berries cool for just a couple of minutes.
Select a large plate to pour the syrup on at the end of this step, and place it in your freezer. Return to the berries, and use a hand immersion blender to blend the ingredients into a lump-free puree. Then pour the mixture immediately through a sieve to remove all the seeds. You should be left with a purple-hued, glossy syrup that is a little thin. When your plate is cold, remove it from your freezer and pour the syrup onto the plate. Slowly rotate the syrup around the cold plate to rapidly cool it down.
Place your plate of syrup in your refrigerator to stay cold while you make the fluff. Whip the heavy cream in a stand mixer at a high speed until it comes to a stiff whipped cream. Whip in the powdered erythritol and vanilla extract, then transfer the whipped cream aside to a clean bowl. Now, add softened cream cheese to the mixer, and blend it with a paddle attachment until the cream cheese is smooth, lump-free, and airy. Add lemon juice and blend again until the consistency is smooth and airy once more.
Blend the whipped cream into the cream cheese until you have a white ‘fluff’. Now, remove the mixing bowl from your stand mixer, and pour the berry syrup over the fluff. Use a spatula to fold the fluff over by hand until the berry syrup is swirled in to your liking. It’s recommended to slightly under-mix the berry syrup into the fluff so you see beautiful contrasting swirls when you scoop the fluff into serving containers. However, you may also incorporate the syrup fully into the fluff for an all-purple, berry-flavored fluff. Each serving is 1 ounce by weight. Serve cold.