Step 1 Make the Mousse: Place the chopped chocolate in a medium bowl and place a fine-mesh sieve on top of the bowl.
Step 2 In a small bowl, whisk together the egg yolks and sugar until pale in color, about 30 seconds.
Step 3 Place ½ cup of the cream in a small saucepan and bring to a simmer over medium-low heat. Slowly whisk the warmed cream into the egg yolk mixture, then scrape the mixture back into the saucepan and return to the heat. Cook, stirring constantly, until thickened and coats the back of a spoon or spatula, about 3 to 5 minutes.
Step 4 Pour the custard mixture through the fine mesh sieve, then stir the white chocolate mixture until completely melted and smooth. Add the Baileys Irish Cream and stir until completely incorporated (it will look loose at first, but will come together). Set aside.
Step 5 Using an electric mixer, whip the remaining 2 cups of heavy cream until it just reaches heavy peaks. Stir one-third of the whipped cream into the custard mixture to lighten it, then gently fold in the remaining whipped cream in two additions, until no white streaks remain.
Step 6 Pour half of the mousse into a 3-quart trifle bowl, top with half of the crushed Oreos, and repeat. Refrigerate for at least 1 hour, or until the mousse is set. Garnish with additional whipped cream, if desired.
