Prepare the graham cracker crust first. Place the cookies into a food processor and pulse into fine crumbs. Add the melted butter and pulse again to coat the cookie crumbs. Transfer the crumbs into a 9-inch spring form pan and use the back of a spoon or potato masher to press the crumbs firmly into the bottom. I also lined my pan with an acetate cake collar to give the pan more height and make it easier to remove the cake from the pan.
Set the crust aside and prepare the cream cheese mousse filling. Into a stand mixer bowl, pour in the chilled heavy cream and add the confectioner's sugar. Whisk the cream on medium speed, then high speed, for 3 to 4 minutes, until stiff peaks form. In a separate bowl, beat the cream cheese using a hand mixer for 2 to 3 minutes, until it's light and fluffy.
Add the melted white chocolate to the cheese and mix again until the mixture is uniform. Add the berry preserves, vanilla extract, lavender oil and purple food coloring. Mix again for a few minutes until the mixture is smooth and creamy.
Next, prepare the agar gelatin. Pour 1/3 cup water into a small saucepan and add the agar gelatin and sugar. Cook the mixture over medium heat for a few minutes. The mixture will thicken into a paste as it cooks. As soon as a thick paste forms, add it immediately into the cream cheese; do not allow it to cool as it will become firm and form clumps. Beat the agar into the cream cheese for a minute.
Add the cream cheese and berry mixture to the whipped cream. Whisk on high speed for about 30 seconds; avoid over-mixing as this may cause the filling to separate. If needed, add more food coloring and fold gently by hand. Pour the filling over the prepared crust and use a small, off-set spatula to level off the top. Press the fresh blackberries into the mousse before placing it into the refrigerator to set.
Allow the mousse to set for an hour or two, then make the lavender jello. Don't make it in advance since it will set at room temperature. In a medium saucepan, combine 2 cups water with the agar gelatin and sugar. Cook the mixture over medium heat for about 3 to 4 minutes, until it begins to simmer. Remove it from heat and add the lavender oil and a tiny bit of purple food coloring. Add the ice water last and stir until the mixture is cooled.
Using a ladle, gently pour the prepared jello mix over the mousse layer. Set the cake back into the refrigerator and allow it to set for at least 4 hours, or leave it to set overnight. Keep the cake in the springform pan and in the refrigerator until ready to serve. To remove the cake from the pan, run a thin spatula along the edge to release the pan.