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 160 to 170 degrees. It takes about 5 to 6 minutes. Let the lemon mixture come to room temperature.

In another bowl, mix together the white chocolate and 1/4 cup heavy cream. Put this bowl over the simmering water. Mix until it melts. Mix until smooth and let come to room temperature.

Whip the remaining 2 2/3 cup heavy cream until stiff peaks form.

Fold about a cup of whipped cream into the white chocolate mixture and the lemon mixture to lighten.

Marry both mixtures and fold in the rest of the whipped cream. Be gentle, you want this to be light and fluffy, not soupy.

Make the Blackberry Lavender Syrup by mixing together the sugar, water, and 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 sit for 20 minutes. Add lemon juice to taste. This syrup will keep for 2 week in the refrigerator.

Layer the mousse with the blackberry lavender syrup and blackberries.

The mousse can be made a day ahead of time, but I would not layer until you are getting ready to serve.