In saucepan, bring milk and cream to boil. Meanwhile, in heatproof bowl, whisk yolks with 3/4 cup/175 mL of the maple syrup; whisk in hot milk mixture. Return to saucepan; cook over medium heat, stirring and scraping bottom constantly, until thick enough to coat spoon (do not simmer), 6 to 8 minutes. Pour through fine sieve into bowl; let cool. Chill in refrigerator.

Meanwhile, in large saucepan over medium-high heat, bring remaining maple syrup to boil; boil over medium heat until reduced by about one-third, 5 to 6 minutes. Remove from heat; when bubbles subside, stir in butter. Let cool. Refrigerate until chilled.

Freeze cream mixture in ice-cream maker according to manufacturer's instructions. Spoon into airtight container; freeze until almost firm. (Or strain into metal cake pan; freeze until almost solid, about 1-1/2 hours, then break up and purée in food processor. Scrape into airtight container.)

Top with maple syrup mixture; with spoon or blade of knife, swirl into ice cream. Freeze until firm.

Makes 6 cups/1.5 L or 12 1/2 cup/125 mL servings.