Prepare the apricot purée: Combine the apricots, granulated sugar, and lemon juice in a small saucepan. Cook over medium heat for 8-10 minutes, stirring occasionally, until the apricots are very soft.

Strain and cool the purée: Transfer the cooked apricot mixture to a blender or food processor and blend until perfectly smooth. Pass the purée through a fine-mesh sieve to remove any remaining skin particles. Set aside to cool to room temperature.

Bloom and dissolve the gelatin: Pour the cold water into a small bowl and sprinkle the gelatin powder evenly over the top. Let it sit for 5 minutes (blooming). Microwave the bloomed gelatin for 10-15 seconds until completely liquid and clear. Let it cool slightly.

Fold in the gelatin: Gently whisk the cooled, liquid gelatin into the cooled apricot purée until fully incorporated. Be careful not to introduce air bubbles.

Whip the cream: In a separate, chilled bowl, beat the heavy whipping cream with the powdered sugar and vanilla extract until medium-stiff peaks form.

Assemble the mousse: Gently fold one-third of the whipped cream into the apricot purée mixture to lighten it. Then, gently fold in the remaining whipped cream in two additions until just combined, making sure not to deflate the volume.

Chill: Carefully spoon or pipe the mousse into individual serving glasses or ramekins. Refrigerate for at least 4 hours, or preferably overnight, until firmly set.