Heat oven to 400 degrees F. Sift together flour, salt, cocoa powder, baking soda, and baking powder. Set aside. Line 2 baking pans with silpats (French nonstick baking mats or parchment paper). In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy. Add egg,s buttermilk, and vanilla. Beat until well combined. Slowly add dry ingredients. Mix until combined.
Using a 1-ounce ice cream scoop, scoop cookies onto lined baking pans, 12 cookies per baking pan. Bake for 12 minutes. Remove to a rack to cool. Repeat with remaining batter. Spread 2 tablespoons of frosting on the bottom of 24 cookies. Sandwich together with remaining 24 cookies. Store in an airtight container up to 3 days.
In a medium heatproof bowl, combine egg whites, sugar, corn syrup, and cream of tartar with 5 tablespoons of cold water. Place over a pot of boiling water. Using a hand held electric mixer beat on high for 7 minutes. Remove from heat and add vanilla extract. Continue to beat until frosting has cooled.
Yield: 3 cups.
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