01 - Combine berries, sugar, and 1 cup water in a saucepan. Bring to a simmer and cook for 5 minutes, gently mashing berries with a spoon. Strain through a fine mesh sieve to remove seeds and pulp; return liquid to saucepan.
02 - Sprinkle gelatin over 1/2 cup cold water in a small bowl and let bloom for 5 minutes. Stir bloomed gelatin into hot berry liquid until fully dissolved. Add lemon juice and mix well.
03 - Pour berry jello into bottom of serving glasses or trifle dish, filling about halfway. Refrigerate for at least 2 hours until firmly set.
04 - Split vanilla bean and scrape seeds into saucepan. Add milk and heat just until steaming. Remove from heat. Whisk egg yolks with sugar until pale, then slowly pour hot milk mixture into yolks while whisking constantly.
05 - Return mixture to saucepan and cook over low heat, stirring constantly, until slightly thickened and coats the back of a spoon. Do not boil.
06 - Sprinkle gelatin over 1 tablespoon cold water in a small bowl; let bloom for 5 minutes. Stir into warm custard until dissolved. Let cool to room temperature, stirring occasionally to prevent skin formation.
07 - Whip cold heavy cream to soft peaks using electric mixer or hand whisk.
08 - Gently fold whipped cream into cooled custard until fully combined and smooth.
09 - Once berry jello is set, spoon or pipe vanilla mousse over the jello layer. Refrigerate for at least 2 more hours until mousse is completely set.
10 - Top with fresh berries and mint leaves before serving, if desired.