Crush 24 Oreo cookies in a food processor until fine crumbs. Melt 6 tablespoons of vegan butter and mix with cookie crumbs until well combined. Press into a 9-inch tart pan and chill in the freezer for 15 minutes.

Blend 1 cup of blackberries and 1 cup of blueberries until smooth. Optionally strain the berry puree through a fine mesh sieve. In a bowl, combine the berry puree with 8 ounces of non-dairy cream cheese, 0.5 cup of sugar, and 1 teaspoon of vanilla extract; mix until creamy.

Whip 1 cup of non-dairy whipping cream on medium-high speed until stiff peaks form, about 3-5 minutes. Set aside while preparing the agar mixture.

Dissolve 1 tablespoon of agar powder in 0.5 cup of non-dairy milk over medium heat, whisking until gently boiling. Cool slightly for 3 minutes. Gently fold this agar mixture into the berry mixture, then fold in the whipped cream.

Pour the mousse filling into the prepared Oreo crust, smoothing the top. Cover with plastic wrap and refrigerate for at least 4 hours (ideally overnight) to set.

Remove from the tart pan, garnish with extra blackberries, slice, and serve chilled.