Preheat oven to 350 F. Line 2, 12-cup muffin tins with baking cups.
In a large bowl sift together flour, coca powder, baking soda, baking powder and salt. Add sugar and stir to combine. In small bowl, whisk together eggs, buttermilk, coffee, oil and vanilla. Divide batter between cups of prepared muffin pans. Bake in preheated oven for 15 to 20 minutes, until a toothpick inserted near the center comes out clean. Allow to set 10 minutes in the pan, then remove from pan and cool on wire cooling rack.
In a saucepan over medium heat, bring an inch of water to rapid simmer. In a heatproof bowl, whisk together egg whites, sugar and cream of tarter. Whisking constantly, place heatproof bowl over simmering water for 3 to 4 minutes, until sugar is dissolved. Remove from heat. Using an electric hand beater or stand mixer fitted with whisk attachment, whip the egg whites to high, stiff and glossy peaks, 5 to 7 minutes. Add vanilla and stir to combine.
Once cool, frost cupcakes with frosting and if desired sprinkle with crushed graham crackers. Makes 24 cupcakes.