Combine first 3 ingredients in a heavy saucepan; whisk in egg yolks and milk. Cook over medium heat, whisking constantly, 5 to 7 minutes or until thickened. Remove from heat; stir in vanilla. Pour into a serving dish; cool. Cover and chill 2 hours.
Beat whipping cream at medium speed with an electric mixer until foamy; gradually add 2 tablespoons sugar, beating until soft peaks form. Spread over custard. Top with fresh blackberries; garnish, if desired.