Place the semi-sweet chocolate chips in a microwave-safe, heatproof bowl. Heat in 30-second increments, stirring thoroughly after each interval, until the chocolate is fully melted and smooth. Set aside to cool slightly.

Once the chocolate has cooled to a warm but not hot temperature, whisk in the egg yolks one at a time. Ensure each yolk is fully incorporated before adding the next. Stir in the unsalted butter until completely melted and combined. Then, whisk in ½ cup of Baileys Irish Cream until the mixture is uniform. Set aside.

In a separate mixing bowl, combine the remaining ½ cup of Baileys with the cold heavy cream. Using an electric mixer on medium-low speed, whisk until the mixture begins to thicken. Add the powdered sugar and increase the speed to medium-high. Continue to beat until soft to medium peaks form. Reserve approximately ¼ of the whipped cream for topping, and gently fold the remaining whipped cream into the chocolate mixture using a spatula.

In a clean, grease-free mixing bowl, whip the egg whites with a pinch of salt until stiff peaks form. This can be done with a hand mixer or a stand mixer fitted with a whisk attachment.

Add a generous spoonful of the whipped egg whites to the chocolate mixture and whisk thoroughly to loosen the texture. Gently fold in the remaining egg whites using a spatula, taking care not to deflate the mixture. Fold until no visible white streaks remain and the mousse is smooth.

Evenly divide the mousse among 6 to 8 serving glasses or dessert bowls. Top each serving with the reserved Baileys whipped cream. Chill the mousse for at least 2 hours, or until set.

Before serving, garnish with chocolate shavings if desired. Serve chilled.