For the cake: Position an oven rack in the center of the oven and preheat to 350 degrees F. Spray and flour three 8-inch cakes pans and line the bottoms with parchment circles.
Combine the chocolate and 4 tablespoons of the butter in the top of a double boiler and heat until melted. Stir until smooth, then stir in the milk. Set aside to cool slightly.
Sift together the flour, baking soda and salt in a bowl.
Whisk the egg whites in a mixer fitted with the whisk attachment to firm peaks. In a new mixer bowl, combine the sugar and remaining 1 1/2 sticks butter. Beat with the paddle attachment on high speed until light and fluffy, 1 to 2 minutes. On medium speed, add the egg yolks one at time and beat until smooth. Beat in the vanilla. Add the chocolate mixture and beat just to combine. Add the flour mixture in 3 additions, alternating with the buttermilk, beginning and ending with the flour. Beat just until the batter is smooth.
Remove the bowl from the mixer. Add the egg whites and fold with a spatula just to incorporate (there should be no streaks of egg whites, but fold gently so as not to deflate too much). Divide the batter among the prepared pans.
Bake until a tester inserted in the center comes out clean, about 30 minutes. Cool on a rack in the pans for about 10 minutes, then remove and cool completely. Remove the parchment.
For the topping: To assemble, set a cake layer on a cake stand flat-side up. Spread 1/2 cup of the jam almost to the edges of the layer. Top with a second cake layer. Spread with the remaining jam and top with the third cake layer, flat-side up. Pour the fudge sauce over the cake and use an offset spatula to cover the top and let it drip down the sides of the cake. Sprinkle toasted coconut on top to garnish right before serving.