In a medium bowl, whisk together the flour, baking soda, and salt. Combine the buttermilk and vanilla in a glass measuring cup.

In the bowl of a stand mixer fitted with a whisk attachment, whisk the whole eggs and egg yolks on medium speed until combined. Add the remaining 1 1/4 cups sugar, increase speed to high, and continue whisking until lightened in color and fluffy, about 2 to 3 minutes. Add the cooled chocolate mixture to the egg mixture and stir on medium until fully incorporated, pausing to scrape down the sides of the bowl.

Add the softened butter, 1 Tablespoon at a time, mixing for about 10 seconds after each addition. Add 1/3 of the flour mixture followed by 1/2 of the buttermilk mixture, mixing until incorporated. Repeat process with remaining flour mixture and buttermilk mixture until batter is thoroughly combined. Scrape down the sides of the bowl and give one last stir before dividing batter equally among the three prepared pans. Lightly tap pans against the countertop to settle any batter and to eliminate any air bubbles.

Bake cakes for 30-34 minutes, or until a toothpick inserted into the center comes out with a few crumbs attached. Remove cakes from the oven and allow to cool in the pan for 15 minutes on a wire rack. Run a knife around the edge of the cakes, carefully flip them out onto a wire rack, peel off the parchment paper, then flip the cakes right side up. Cool completely.

Microwave the chocolate in a medium microwave-safe bowl until smooth. Set aside and allow to cool while you whip the cream.

Place heavy cream and sugar in a well-chilled bowl of a stand mixer fitted with a well-chilled whisk attachment. Whisk on high until stiff peaks form, about 1 1/2 minutes.

Stir 1/4 of the whipped cream into the melted chocolate until smooth and completely incorporated. Add the combined whipped cream and chocolate mixture to the remaining whipped cream and use a rubber spatula to gently fold together.

In a microwave-safe bowl, combine the heavy cream and butter and heat in the microwave until bubbling, about 2 minutes.

Remove hot cream mixture from the microwave and immediately dump chocolate in, assuring that all of the chocolate is covered in the cream. Cover with plastic wrap and let stand for 5 minutes.

Remove plastic wrap and stir until smooth and silky. Allow ganache to sit at room temperature for 1 hour, until thickened and spreadable.

Slice each cake layer in half horizontally, so that you end up with 6 cake layers total. Place one cake layer on a plate or cake pedestal and spread with 1/6 of the mousse filling. Repeat, alternating layers of cake and filling, ending with a layer of mousse. Slowly pour ganache in the center of the cake, allowing it to drip down the sides. Store in the refrigerator until ready to serve.