To make the sauce: Combine granulated sugar, lemon juice and water in a small heavy saucepan. Bring to a boil over medium-high heat, stirring occasionally. Cook, without stirring, until the syrup turns deep amber, 10 to 15 minutes. Remove the caramel from the heat and let cool for 2 minutes. Whisk in butter until it is incorporated, then gradually whisk in 3/4 cup evaporated milk. Return to low heat and stir until the caramel has dissolved completely. Whisk in vanilla. Let cool slightly before serving.