1. Stir together 2 1/2 cups of the milk and the granulated sugar in a large saucepan. Heat until just steaming. Remove from heat; set aside.
2. Meanwhile, in a medium-size bowl, mix together dark-brown sugar, cornstarch and nutmeg until evenly blended. Whisk in egg yolks, remaining 1/2 cup milk and salt. Stir a little of the hot milk mixture into the brown sugar mixture to warm egg yolks, then stir brown sugar mixture back into milk mixture in saucepan. Cook, stirring constantly with wooden spoon, over medium-low heat until thickened and it registers 160 degrees on an instant-read thermometer, 20 minutes. Remove from heat. Add butter and vanilla; stir until smooth.
3. Pour mixture through strainer into six dishes. Cool 10 minutes. Cover surface directly with plastic; refrigerate at least 2 hours. Garnish with whipped cream.