Bring maple syrup to a boil in a saucepan; cook for 5 minutes.
Pour 3/4 of the cream in a separate saucepan; bring to a boil. Gradually stir maple syrup into cream.
Stir remaining cream with cornstarch in a bowl until smooth; add to the maple mixture. Stir in butter. Reduce heat to low and gently simmer until slightly thickened, about 10 minutes. Transfer to a small fondue pot.