01 - Blend raspberries with granulated sugar and lemon juice until completely smooth. Press mixture through a fine-mesh sieve to eliminate all seeds. Reserve puree for later use.
02 - Submerge gelatin sheets in cold water for 5 minutes to soften. If using powdered gelatin, sprinkle over 2 tbsp cold water and let stand until absorbed.
03 - Warm 2 tbsp raspberry puree in a small saucepan over low heat. Drain gelatin sheets thoroughly and stir into warm puree until dissolved. For powdered gelatin, stir until completely dissolved. Combine mixture with remaining puree and cool to room temperature.
04 - Using a hand or stand mixer, whip cold heavy cream with vanilla extract until medium peaks form. Transfer whipped cream to refrigerator until needed.
05 - In a clean, dry bowl, beat egg whites until soft peaks appear. Gradually sprinkle in powdered sugar while continuing to whip until stiff, glossy peaks form.
06 - Gently fold cooled raspberry puree into whipped cream using a spatula, stopping when nearly incorporated. Lightly fold in meringue until mixture is uniformly smooth and no white streaks remain.
07 - Divide mousse evenly among serving glasses or pipe using a pastry bag. Refrigerate uncovered for at least 2 hours until completely set and firm.
08 - Top each serving with fresh raspberries. Optionally dust with powdered sugar or add fresh mint leaves just before serving.