01 - Preheat oven to 350°F. Line a 9x13-inch baking sheet with parchment paper.
02 - Beat eggs and sugar with electric mixer until pale and thick, about 5 minutes. Gently fold in flour, vanilla, and salt until just combined. Spread batter evenly on prepared sheet.
03 - Bake for 10-12 minutes until lightly golden and springy to the touch.
04 - Let sponge cool completely, then cut out 8 circles using a 2.5-inch round cookie cutter. Place each circle at the bottom of individual silicone molds or lined muffin tins.
05 - Soften gelatin sheets in cold water for 5 minutes. If using powdered gelatin, bloom as directed on package.
06 - Combine blackberries, sugar, and lemon juice in saucepan. Simmer over medium heat until berries are soft, 5-7 minutes. Puree and strain through fine sieve to remove seeds.
07 - While puree is still warm, add softened gelatin and stir until completely dissolved. Allow mixture to cool to room temperature.
08 - Whip cold heavy cream to soft peaks using electric mixer or whisk.
09 - Gently fold cooled blackberry mixture into whipped cream until smooth and uniform.
10 - Spoon or pipe mousse over each sponge base in molds. Smooth tops evenly. Refrigerate for at least 4 hours until completely set.
11 - Bloom gelatin as above. Gently heat blackberry puree and sugar in small saucepan until sugar dissolves and mixture is warm, not boiling. Stir in gelatin until dissolved. Cool until slightly thickened but still pourable.
12 - Unmold chilled mousse cakes and place on rack over tray. Pour glaze over tops, allowing it to drip down sides. Chill briefly to set glaze.
13 - Garnish with fresh blackberries and edible flowers or mint leaves if desired. Serve chilled.