1 1/2 cups sugar
1/4 cup water
1 1/2 teaspoons fresh lemon juice
1 cup whipping cream
2 tablespoons (1/4 stick) unsalted butter
Stir sugar, water, and lemon juice in heavy medium saucepan over low heat until sugar dissolves.
Increase heat; boil without stirring until syrup is deep amber color, about 7 minutes.
Remove from heat. Add in cream (mixture will bubble vigorously).
Return to low heat; stir until any bits of caramel dissolve.
Add butter; whisk until smooth. (Can be made 1 day ahead. Cover and chill.)