Butter a 9-inch and 6-inch (or two 8-inch) cake pans with removable bottoms. Dust with flour and shake off excess.
Sift together 1 ⅓ cups all-purpose flour, ½ cup unsweetened cocoa powder, ¾ teaspoon baking soda, ½ teaspoon baking powder and ¼ teaspoon salt.
In a large bowl beat 10 tablespoons unsalted butter until creamy, about 1 minute. Add ½ cup brown sugar and ½ cup white sugar gradually and beat for 2 minutes after adding the last part.
Add 3 eggs, one at a time, beating well after each addition.
Add 1 teaspoon vanilla extract and 2 ounces unsweetened chocolate. The mixture will look curdled, that’s fine.
Add the dry ingredients in 3 parts, alternating with ½ cup buttermilk in 2 parts. Mix just until well combined.
Slowly add ½ cup boiling water and mix just until combined.
Add 4 ounces milk chocolate chips with a spatula, and divide between pans, using a bit more in the large one.
Bake for about 40 to 45 minutes, until a tester inserted in the middle of both cakes, comes out clean.
Let cool completely on a wire rack before filling and frosting. It can be made the day before and kept wrapped in plastic.
For the meringue:
Put a pan with water over medium heat.
On top of it, place the glass or metal bowl from the mixer with 4 egg whites and 1 cup sugar. The water should not touch the bottom of the pan.
Mix constantly with a hand whisk. The water will start to boil. Don't let water get into the bowl. The mixture will be thick at first but will liquefy as it warms. This is your cue to remove it. It will also be warm to the touch (use your pinky finger to test).
Transfer to the mixer (or mix with a hand-held electric beater) and whip until stiff and glossy, about 5 minutes. You should have rather stiff peaks.
To assemble the cake:
Cut both cakes in half. If you want to, dab the cake plate with a bit of dulce de leche in the middle and place one of the large layers on top; that way the cake won’t move.
Fill with a thick layer of dulce de leche, about ¾ of the amount, spreading evenly but not reaching the edges completely. Top with the other large layer, pressing gently. The dulce de leche will fatten towards the edges.
Add some meringue on top of the cake making a circle as big as the layers that you haven’t used yet. Place one of those layers on top, and fill with meringue, not reaching the edges completely.
Mix the rest of the dulce de leche with the meringue you have left, swirling just a few times to have a marble effect. (As you can see I didn’t quite get that effect, but not for lack of trying…).
Add a thin layer of meringue to all of the cake and then add a thicker one. However, you can. The meringue is soft for having a perfect frosting.
Sprinkle with the chocolate crumbs.
Notes
Cake: the layers can be made in advance and kept, well wrapped, in the refrigerator for 3 days or frozen for up to a month.Cake pans: the recipe is enough for 3 8 or 9-inch layers, the former will be a bit taller. The filling and frosting will be enough if you use those pans.Meringue: I make swiss meringue because it’s quite easy and stays rather soft, which I particularly like. This makes it harder to have a perfect frosting, which I don’t mind, but if you do, consider making Italian meringue (which involves syrup and a thermometer).