Cut white chocolate into small pieces. Pierce the lemon several times with a fork. And squeeze out lemon juice.

Beat egg lightly until it gets watery. Add granulated sugar and mix well for about 30 seconds. Strain the lemon juice and add it to the egg mixture. Mix well for about 30 seconds.

Put the mixture into a pot. Boil water in a pan. Put the pot in hot water and heat the mixture while stirring on low heat.

Continue to stir until it starts to thicken. Turn off the heat and put the white chocolate. Wait for 30 seconds as it is and mix well until it gets smooth. Put the pot out from the hot water.

Warm the milk in the hot water on low heat until it reaches body temperature. Add the warmed milk to the lemon mixture in 3 parts. Mix well each time.

Add granulated sugar to heavy cream and whip it while cooling with ice water. Add vanilla essence. Whip it until soft peaks hang down.

Put the heavy cream out from the ice water. Cool the pot (the lemon mixture) in the ice water for 5 minutes. Strain it.

Add the whipped heavy cream to the lemon mixture in 3 parts.
Mix well each time until it gets smooth.

Pour it into a container. Let it cool in a freezer. Mix it roughly when 3 hours have passed. Let it cool for another 3-4 hours. It's all done!