Heat cream, condensed milk and lemon grass in a saucepan over medium heat until mixture steams. Remove from heat before the mixture boils and allow to steep for at least 30 minutes, up to 1 hour and 30 minutes.
Reheat the cream mixture over medium heat until it begins to steam. Whisk egg yolks in a small bowl. Temper the egg yolks by pouring a small amount of the hot mixture in while whisking constantly to avoid scrambling the eggs. Pour the warm egg mixture into the cream mixture. Continue to cook and stir until the mixture is able to coat the back of a spoon. Pour the mixture through a sieve to remove the bits of lemon grass. Refrigerate until cold.
Pour the chilled mixture into an ice cream maker and freeze according to manufacturer's directions until it reaches "soft-serve" consistency. Transfer ice cream to a 1- or 2-litre lidded plastic container; cover surface with cling film and seal. For best results, ice cream should ripen in the freezer for at least 2 hours or overnight.