Add the sliced strawberries and the sugar to a blender or food processor and puree.: The moment you start pureeing you'll smell that concentrated, ripe strawberry aroma that signals flavor depth, and the mixture should look glossy and evenly pink. A smooth puree ensures the mousse will be silky, so pause to scrape the sides if needed and blitz until there are no visible chunks. Avoid overheating the puree by pulsing rather than running the motor continuously, since warm fruit can make the whipped cream looser later on.
Remove 1/2 a cup of puree and set aside.: The reserved half cup offers a vivid layer at the bottom of each serving, and separating it now keeps its bright color and texture intact. You will notice the puree is slightly aerated and fragrant when scooped, and setting it aside prevents it from being overworked. Be careful not to mix this reserved portion back into the main batch, or the layered presentation will be lost.
In a medium bowl, beat the cream cheese and vanilla with a hand mixer until smooth. Add in the strawberry puree, not the half cup set aside, and mix until combined.: At this stage the bowl will give off a creamy, slightly tangy scent as the softened cream cheese loosens and becomes velvety. The texture should be lump free, glossy, and homogenous once the puree is folded in. This integration matters because the cream cheese stabilizes the mixture and keeps the mousse from becoming watery. If the mixture appears grainy or has lumps, continue beating gently until smooth, but don't over mix which can deflate air pockets you want to preserve.
In a cold bowl add the heavy whipping cream and beat with a hand mixer until stiff peaks form. Gently fold into the cream cheese mixture.: As you whip the chilled heavy whipping cream you'll hear a subtle change in sound from a liquid swish to a thicker, almost billowy rhythm, and visually you'll see peaks that hold their shape. Folding this whipped cream into the denser fruit and cream cheese base introduces air essential for the mousse texture. Use gentle strokes and a flexible spatula so you keep the volume, and stop folding once the mixture looks uniform. A common mistake is over folding which yields a denser result instead of an airy mousse.
Divide the 1/2 cup of reserved puree evenly between the 6 small/medium glasses and top with the strawberry mousse.: The reserved puree should be glossy and vividly colored, making an attractive first layer in each glass. Spoon or pipe it gently so it settles flat, then add the mousse on top so you get a pretty contrast and the flavor interplay of a concentrated fruit base with creamy mousse above. Pressing too hard when adding the mousse can mix the layers, so be gentle to maintain a clean look and distinct mouthfuls of flavor.
Refrigerate for approximately 1 hour or even overnight if desired. Top with fresh sliced strawberries and serve.: During chilling the mousse firms and the flavors marry, producing a creamier, more cohesive texture and an intensified strawberry aroma. One hour gives a pleasant set while overnight deepens the flavors and makes serving easier for gatherings. When you top with fresh strawberries just before serving the fruit stays bright and adds a fresh note. Avoid leaving assembled, uncovered servings in the fridge to prevent absorption of other odors or a dried surface.