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