Place egg whites and sugar in the bowl of a stand mixer and whisk until combined.
Place the mixing bowl over a hot water bath on the stove and whisk constantly by hand until the mixture is no longer grainy to the touch (approx. 3mins). You can feel with your fingers if it is still grainy or not.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled. The bowl will be no longer warm to the touch (approx. 5-10mins). Don't worry, you can't over whisk the meringue.
Switch to paddle attachment on the stand mixer.
Slowly add barely softened cubed butter one cube at a time and mix until smooth.
Add vanilla, melted chocolate, and espresso powder. Whip until smooth.
Scrape the filling off of 3 or 4 Oreo cookies. Put the cookies in a plastic bag and pound with a mallet or rolling pin until you have small crumbs. You can also pulse them in a food processor to get the same result, but don't grind them too fine.
Mix the cookie crumbs into the meringue until combined evenly.
Frost cake and refrigerate before adding ganache and topping.