Chill six 8 or 9 ounce stemmed dessert glasses.

In the top of a double boiler set over simmering water, whisk together lemon zest, lemon juice, one whole egg, 1 egg white, Baileys Irish Cream and 3/4 cup of the sugar.

Cook, stirring continuously, for 12-15 minutes, or until the mixture thickens into a custard.

Remove the double boiler from the heat and stir in the butter until melted and combined.

Set the top of the double boiler in a large bowl filled with ice water, let the mixture cool, whisking occasionally, until cooled.

In a separate small bowl, combine the remaining 4 egg whites and the cream of tartar.

Beat with an electric mixer until soft glossy peaks form.

Add the remaining 1/4 cup of sugar, a little at a time, and continue to beat until stiff peaks form.

Stir a few teaspoonsful of the egg white mixture into the cooled custard to lighten it.

Gently fold in the remaining egg whites and 1 1/2 cups of the blueberries.

Spoon the mousse mixture into the prepared glasses.

Refrigerate for 1 to 2 hours, or until set.

At serving time, garnish with the reserved blueberries.