In medium saucepan, whisk milk and sugar over medium heat until sugar is dissolved, 1 to 2 minutes. In a large bowl, whisk egg yolks. Whisking constantly, pour hot mixture into yolks in a slow and steady stream.
Return mixture to pan; cook over medium-low heat, stirring often, until thick enough to coat back of spoon, 20 to 25 minutes; do not let simmer. Quickly strain into a bowl. Stir in bourbon, if desired, and cream. Cool completely; cover with plastic wrap; refrigerate until chilled. Garnish with nutmeg, if desired.