In a saucepan over medium heat, combine the blackberries and sugar. Stir gently until the blackberries start to release their juices, about 5 minutes.

Add the brandy and lemon juice to the saucepan, stirring to combine. Allow the mixture to simmer for another 5-7 minutes until the blackberries are soft.

If you prefer a thicker sauce, mix the cornstarch with water in a small bowl and add it to the saucepan. Stir continuously until the sauce thickens, about 2-3 minutes.

Remove the saucepan from heat and let the sauce cool slightly before serving.