This is an oreo cookie buttercream icing, so I would use whatever buttercream icing you prefer, this looks like a relatively decent one:
Oreo buttercream frosting (see note)
adapted from Nick Malgieri’s Perfect Cakes
2 large egg whites
1/2 cup sugar
12 tbsp of unsalted butter, softened
6 oreo cookies, crushed
6 oreo cookies, halved for decoration (optional)
1. Whisk the egg whites and sugar together in the bowl of an electric mixer. Set the bowl over simmering water and whisk gently until the sugar is dissolved and the egg whites are hot (approximately 160 degrees).
2. Attach the bowl to the mixer and whip with the whisk on medium speed until cooled. Switch to the paddle and beat in the softened butter and continue beating until the buttercream is smooth. Dissolve the instant coffee in the liquor and beat into the buttercream.