Bring 1 cup sugar and 2 teaspoons lemon juice to a boil in a saucepan over medium-high heat, stirring to dissolve the sugar. Continue to cook, swirling the pan but not stirring, until amber. Remove from the heat and carefully stir in 3/4 cup heavy cream and 2 tablespoons bourbon. Return to the heat and whisk until smooth; swirl in 2 tablespoons butter. Put 2 scoops vanilla ice cream in each glass and fill with root beer; drizzle with the caramel sauce.
Photograph by Antonis Achilleos