Carefully separate the yolks from the whites, ensuring no yolk gets into the whites to help them whip properly later.

In a mixing bowl, combine the egg yolks and sugar. Whisk vigorously until the mixture turns pale and creamy. This ensures the sugar dissolves and incorporates evenly.

Gradually add the cornstarch to the yolk mixture, whisking to avoid lumps. Slowly pour in the water while stirring to create a smooth consistency.

Juice 1.5 lemons and grate the zest from a small lime. The fresh citrus will give the mousse its vibrant flavor.

Add the lemon juice and lime zest to the yolk mixture. Mix well until the ingredients are fully incorporated and smooth.

Transfer the mixture to a saucepan and place it over low heat. Stir constantly with a wooden spoon or spatula to prevent sticking or curdling. Cook until the cream thickens enough to coat the spoon evenly.

Remove the saucepan from the heat. Pour the cream into a bowl, add the butter, and whisk quickly to create a glossy and silky texture.

Cover the surface of the cream with cling film to prevent a skin from forming. Let it cool completely to room temperature.

Using a clean, dry bowl, beat the egg whites until they form stiff peaks. This will add lightness to the mousse.

Gently fold a small portion of the whipped egg whites into the cooled lemon cream to loosen it. Then, gradually fold in the rest, being careful not to deflate the mixture.

Spoon the mousse into small serving glasses or bowls. Smooth the tops if needed and refrigerate for at least 2 hours to allow the mousse to set.

Just before serving, top the lemon mousse with whipped cream for a finishing touch. Optionally, garnish with lemon zest or a sprig of mint for extra flair.