Oreo-Stuffed Cupcakes With Marshmallow Frosting

Preheat oven to 350 F. Grease a 12-count muffin tin or line it with muffin liners. Place an Oreo cookie in the bottom of each liner.
In a large bowl, sift flour, baking powder, baking soda and salt. Mix well.
In the bowl of an electric mixer fitted with the paddle attachment, mix eggs and sugar until well combined. Gradually add oil and vanilla, mix until just blended.
With the mixer on low, slowly add half the flour mixture; mix until just blended. Add the buttermilk, mix until just blended. Slowly add the remaining flour mixture, scraping down sides of the bowl with a spatula, as necessary, until just blended.
Divide batter evenly among prepared muffin cups. Transfer muffin tin to oven and bake for 18 to 20 minutes. Transfer muffin tin to a wire rack and let cupcakes cool completely.
To prepare the frosting, in a large heatproof bowl, combine sugar and egg whites. Using a handheld electric mixer, beat on high speed until foamy, for a minute or so. Set bowl over a pan of barely simmering water. Beat on high speed until frosting forms stiff peaks, about 10 minutes.
Remove from heat, stir in vanilla and water, and beat for another 2 minutes until frosting thickens. Transfer frosting to a large pastry bag fitted with a wide plain pastry tip and pipe-frost the cupcakes. The frosting will harden slightly as it cools.
Place an Oreo cookie on top of the frosting. Sprinkle a teaspoon of cookie crumbs before serving.