In a small, heavy-bottomed saucepan, combine the lemon juice, yolks, and sugar. Cook, stirring, over medium-low heat, until the mixture thickens, 5 to 6 minutes. Remove from heat and pour through a fine strainer into a large bowl to remove any lumps. Add the butter and stir until melted. Cover with plastic wrap, pressing the plastic against the surface of the mixture to eliminate any air bubbles. Refrigerate until chilled, about 30 minutes.
Meanwhile, beat the cream with a whisk or an electric mixer to form stiff peaks. Fold the whipped cream into the chilled lemon mixture until well combined. Divide into a muffin tin with paper liners or six 4-ounce ramekins and freeze for 2 hours.