Halve 3 lemons and scoop out the pulp.

Squeeze out the juice.

Remove the zest from 1 lemon.

In a saucepan, combine 180 ml water, 120 ml lemon juice, 150 g sugar, 30 g cornstarch, 100 g condensed milk, 3 egg yolks, and lemon zest.

Heat over low heat, stirring constantly, until thickened.

Remove from heat and stir in 50 g soft butter until well combined.

Allow to cool at room temperature.

Divide the lemon mousse into the lemon halves.

Refrigerate for 2 hours to stabilize.