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 to 12 minutes until lightly golden and springy to touch. Cool completely.
04 - Cut 8 circles using a 2.5-inch round cookie cutter. Place each circle at bottom of silicone molds or lined muffin tins.
05 - Soften gelatin sheets in cold water for 5 minutes or bloom powdered gelatin according to package directions.
06 - Combine blackberries, sugar, and lemon juice in saucepan. Simmer over medium heat until berries are soft, 5 to 7 minutes.
07 - Puree cooked blackberries and strain through fine sieve to remove seeds.
08 - Stir softened gelatin into warm puree until dissolved. Cool to room temperature.
09 - Whip heavy cream to soft peaks.
10 - Gently fold cooled blackberry mixture into whipped cream until smooth and uniform.
11 - Pipe or spoon mousse over each sponge base in molds. Smooth tops. Refrigerate at least 4 hours until set.
12 - Bloom gelatin as above. Heat blackberry puree and sugar in small saucepan until sugar dissolves and mixture is warm. Stir in gelatin until dissolved. Cool until slightly thickened but pourable.
13 - Unmold chilled cakes onto rack over tray. Pour glaze over tops, allowing to drip down sides. Chill briefly to set.
14 - Top with fresh blackberries and edible flowers or mint. Serve chilled.