Whisk together the egg yolks, sugar, lemon juice, zest, and salt.
Put the egg yolk mixture over a bowl of simmering water. Whisk the mixture until it thickens and a thermometer reads 170°F. It takes about 5 to 6 minutes. Let the mixture cool to room temperature.
In the meantime, mix together the white chocolate and 1/4 cup of heavy cream in another bowl. Put this bowl over the simmering water. Stir until the chocolate melts and mixture is smooth, about 3 minutes. Cool to room temperature.
Whip the rest of the heavy cream until stiff peaks form.
Divide 1 cup of whipped cream between the white chocolate mixture and the lemon mixture, folding it in gently.
Combine both mixtures and fold in the rest of the whipped cream. Be gentle, you want this to be light and fluffy.
To Make The Blackberry-Lavender Syrup:
Mix the sugar, water, and dried lavender in a large microwave-safe bowl. Microwave on high for about 3 to 4 minutes, or until the sugar is dissolved. Whisk in the blackberry jam. Let the mixture stand for 20 minutes. Add lemon juice to taste.
To serve, layer the mousse with the blackberry-lavender syrup and the fresh blackberries.