Beat the cream with a stand mixer (fitted with the whisk beater) or electric hand mixer (also fitted with the whisk beaters) on medium-high speed until stiff peaks have formed. That'll take about 2-3 minutes.
Place the whipped cream in another bowl. Set aside until ready to use.
Remove the whisk attachment from the mixer and put on the paddle attachment. Add the cream cheese and sour cream to the bowl that you just used to beat the cream (you don't need to clean it).
Mix at medium-high speed until totally smooth and well-combined (about 1 minute).
With the mixer on low, gradually add 3/4 cup powdered sugar. Taste and if it's pleasantly sweet (keeping in the mind that once you add the whipped cream, it'll taste less sweet), continue to the next step. If you want if sweeter, add the remaining 1/4 cup.
Once all of the powdered sugar has been incorporated, pour in the vanilla and add the salt and beat at medium until the mixture is totally smooth and light (about 2 minutes).
Add the whipped heavy cream to the bowl and use a silicone spatula to carefully fold it into the cream cheese mix.
Transfer to serving dishes and refrigerate until ready to serve. Top with garnish, if desired, right before serving.
Refrigerate leftover mousse in an airtight container for up to two days.