In a bowl of a standing mixer, pour the sugar then add the egg whites and cream of tartar. Use a hand whisk to blend it all.
Place the bowl on top of a pot with simmering water on the stove. Make sure the water does not touch the bottom of the bowl.
Use a hand whisk to stir the mix and cook until it reaches 130F or when the sugar has dissolved.
Remove bowl from stove and back to the mixer fitted with the whisk attachment.
Mix until a stiff meringue has formed and the bottom of the bowl is cool to the touch of your hand.
Reduce speed to low-medium and add the butter a piece at a time, gradually increase speed to medium-high and mix until a smooth creamy buttercream has formed.
Measure 1 cup of buttercream and place in a medium bowl
Separate the Oreos cookies remove and place the filling in the bowl with the small amount of buttercream.
Use a fork or a hand whisk to blend the filling and buttercream then in a piping bag fitted with a round tip.
Place the separated cookies in a food processor and process it into a very fine meal. Remove and lumps or large pieces you notice.
Sprinkle the ground Oreos on top of the buttercream and mix until a cohesive buttercream has formed.
Fill a large piping bag fitted with a large tip (your choice of shape).