To make the cake, preheat oven 350 F. Grease a 9x13-inch baking pan, and then line with parchment paper. Grease parchment paper; set aside.
Place chocolate and cocoa powder in small, heatproof bowl.
Pour boiling water over the chocolate and cocoa powder; stir until smooth.
Whisk flour, baking soda, and salt together in a medium bowl. Set aside.
Beat coconut oil and sugars on medium-high until light and fluffy, about 3-4 minutes.
Beat in the egg yolks, one at a time, until just incorporated.
Reduce speed to low, and beat in chocolate mixture and vanilla.
Stir in half of the flour mixture, the buttermilk, and then the other half of the flour mixture, stirring in each until just combined.
In a separate bowl, beat egg whites on high until they form stiff peaks, about 4-5 minutes.
Gently fold in beaten egg whites.
Pour batter into prepared pan.
Bake about 20-30 minutes, or until cake springs back when lightly pressed and a toothpick inserted in the center comes out clean.
Cool completely in pan.
To make the coconut pecan topping, combine cream, cornstarch, sugar, egg yolks, and butter. Make sure that the cornstarch is fully incorporated before you begin cooking the mixture.
Cook over medium heat, stirring occasionally, until it begins to thicken and bubble, about 10 minutes.
Cook, whisking the entire time, for 2 minutes more. Be sure to keep whisking because it can easily scorch at this point.
Remove from heat.
Fold in vanilla, coconut, and pecans, and then cool until thickened, about 30 minutes.
To make the ganache, place chocolate in a heat-proof bowl.
Bring cream to a simmer in a small saucepan.
Pour cream over chocolate, and let it sit, covered, for 5 minutes without stirring.
Uncover, and whisk until melted and smooth.
To put it all together, lift cake out of the pan.
Using a serrated knife, slice cake horizontally into 3 equal-sized rectangles about 4 1/2x9-inches each.
Coat the first layer of cake with 1/3 of the ganache, then cover evenly with 1/3 of the coconut pecan filling.
Repeat with remaining cake layers, ganache, and filling.
Leave sides of cake naked.
Let cake set for 1-2 hours before slicing.
Cover, and refrigerate any leftover cake.