In a deep saucepan, boil maple syrup to 230° F on a candy thermometer.
In a medium-size bowl, beat egg yolks until light.
Pour hot (but not boiling) maple syrup in a very fine stream into egg yolks while beating constantly.
Continue to beat until mixture is very thick.
Beat in lemon juice and rum extract.
Beat 2 minutes longer (until mixture has cooled a bit).
Transfer egg yolk mixture to a large bowl, and fold in whipped cream.
Freeze until firm.