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Center a rack in the oven and preheat to 350 degrees.
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Butter two 8-by-2-inch round cake pans, dust the insides with flour, tap out the excess and line the bottoms with parchment or wax paper.
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Put the pans on a baking sheet.
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To make the cake: Sift together the flour, cocoa, baking soda, baking powder and salt.
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Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter on medium speed until soft and creamy.
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Add the sugars and continue to beat for another 3 minutes.
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Add the eggs one by one, beating for 1 minute after each addition.
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Beat in the vanilla; dont be concerned if the mixture looks curdled.
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Reduce the mixer speed to low and mix in the melted chocolate.
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When it is fully incorporated, add the dry ingredients alternately with the buttermilk, adding the dry ingredients in 3 additions and the milk in 2 (begin and end with the dry ingredients); scrape down the sides of the bowl as needed and mix only until the ingredients disappear into the batter.
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At this point, the batter will be thick, like frosting.
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Still working on low speed, mix in the boiling water, which will thin the batter considerably.
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Switch to a rubber spatula, scrape down the bowl and stir in the chopped chocolate.
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Divide the batter evenly between the two pans and smooth the tops with the rubber spatula.
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Bake for 25 to 30 minutes, rotating the pans at the midway point.
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When fully baked, the cakes will be springy to the touch and a thin knife inserted into the centers will come out clean.
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Dont worry if the tops have a few small cracks.
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Transfer the cake pans to a rack and cool for about 5 minutes, then run a knife around the sides of the cakes, unmold them and peel off the paper liners.
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Invert and cool to room temperature right side up.
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(The cooled cake layers can be wrapped airtight and stored at room temperature overnight or frozen for up to 2 months.)
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When you are ready to fill and frost the cake, inspect the layers.
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If the cakes have crowned, use a long serrated knife and a gentle sawing motion to even them.
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With the same knife, slice each layer horizontally in half.
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Set 3 layers aside and crumble the fourth layer; set the crumbs aside.
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To make the filling and frosting: Put the egg whites in a clean, dry mixer bowl or in another large bowl.
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Have a candy thermometer at hand.
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Put the sugar, cream of tartar and water in a small saucepan and stir to combine.
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Bring the mixture to a boil over medium-high heat, cover the pan and boil for 3 minutes.
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Uncover and allow the syrup to boil until it reaches 242 degrees on the candy thermometer.
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While the syrup is cooking, start beating the egg whites.
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When the syrup is at about 235 degrees, begin beating the egg whites on medium speed with the whisk attachment or with a hand mixer.
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If the whites form firm, shiny peaks before the syrup reaches temperature, reduce the mixer speed to low and keep mixing the whites until the syrup catches up.
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With the mixer at medium speed, and standing back slightly, carefully pour in the hot syrup, pouring it between the beater(s) and the side of the bowl.
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Splatters are inevitable dont try to scrape them into the whites, just carry on.
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Add the vanilla extract and keep beating the whites at medium speed until they reach room temperature, about 5 minutes.
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You should have a smooth, shiny, marshmallowy frosting.
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Although you could keep it in the fridge in a pinch, its really better to use it right now.
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To assemble the cake: Put a bottom layer cut side up on a cardboard cake round or on a cake plate protected by strips of wax or parchment paper.
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Using a long metal icing spatula, cover the layer generously with frosting.
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Top with a second layer, cut side up, and frost it.
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Finish with the third layer, cut side down, and frost the sides and top of the cake.
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Dont worry about smoothing the frosting it should be swirly.
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Now, cover the entire cake with the chocolate cake crumbs, gently pressing the crumbs into the filling with your fingers.
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Refrigerate the cake for about 1 hour before serving.
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(If its more convenient, you can chill the cake for 8 hours or more; cover it loosely and keep it away from foods with strong odors.)
