Preheat the oven to 350°F (175°C) and grease two 8-inch cake pans.
In a mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
In another bowl, beat the butter and sugar with an electric mixer until light and fluffy.
Add the egg whites one at a time, mixing well after each addition.
Stir in the vanilla extract.
Gradually add the dry ingredients, alternating with the buttermilk, mixing until just combined.
Fold in the crushed chocolate sandwich cookies.
Divide the batter evenly between the prepared cake pans and smooth the tops.
Bake for 28-30 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool in the pans for 10 minutes, then transfer to a cooling rack to cool completely.
For the frosting, fold the crushed chocolate sandwich cookies into the buttercream frosting.
Spread a layer of frosting over one cake layer, then place the second cake layer on top.
Frost the entire cake with the cookies and cream buttercream, smoothing the edges.
Garnish with whole chocolate sandwich cookies on top before serving.