In a medium saucepan, combine fresh blackberries and lemon juice over medium heat. Stir frequently for about 20 minutes until it reduces to approximately 1/3 cup.

Pour the blackberry mixture through a fine mesh sieve to remove the seeds. Allow the strained puree to cool completely.

In a large mixing bowl, beat softened butter and cooled blackberry puree together until fully blended.

Gradually sift in powdered sugar, beating until fluffy and smooth, about 3-5 minutes.

Add vanilla extract and adjust consistency with milk or cream until desired thickness is achieved.