Combine cornstarch and brown sugar in heavy 2-quart saucepan; stir until well mixed. Add milk and half & half; stir until well mixed.
Cook over medium heat, stirring constantly and scraping bottom of pan, 12-14 minutes or until mixture thickens and begins to boil. Remove from heat.
Beat egg yolks in bowl. Very slowly pour 1 1/2 cups hot milk mixture into egg yolks, whisking constantly. Slowly pour egg mixture back into hot milk mixture in saucepan, whisking constantly.
Cook over medium-low heat, whisking constantly, 2-3 minutes or until mixture thickens and begins to boil. Immediately, remove from heat. Stir in butter and French vanilla flavoring blend.
Divide custard among 6 individual serving dishes. Cover; refrigerate at least 30 minutes or until serving time. Garnish with fresh berries and mint leaves, if desired.
*Substitute 2 teaspoons vanilla.
It is important to slowly add the hot milk mixture to egg yolks and stir constantly to prevent eggs from curdling.