01 - Preheat oven to 350°F. Line a 9x13-inch baking sheet with parchment paper.
02 - 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.
03 - Bake for 10-12 minutes, until lightly golden and springy. Let cool completely.
04 - 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 or bloom powdered gelatin as directed.
06 - 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.
07 - While puree is warm, add softened gelatin and stir until dissolved. Allow to cool to room temperature.
08 - Whip heavy cream to soft peaks. Gently fold cooled blackberry mixture into the whipped cream until smooth and uniform.
09 - Spoon or pipe mousse over each sponge base in the molds. Smooth tops. Chill in the refrigerator for at least 4 hours, or until set.
10 - Bloom gelatin as above. 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.
11 - Unmold chilled mousse cakes and place on a rack over a tray. Pour glaze over the tops, allowing it to drip down the sides. Chill briefly to set.
12 - Garnish with fresh blackberries and edible flowers or mint, if desired. Serve chilled.