Preheat oven to 350°F.
Spray 2 9-inch cake pans with cooking spray. Line with parchment paper that has been cut into a circle to fit the bottom of the pan. If desired, spray again, all over the inside and then sprinkle with a little flour.
Sift together the flour, baking soda, and salt. Set aside.
Pour the boiling water into a heat-proof bowl along with the sweet chocolate chunks. Use a whisk or spoon and stir until the chocolate has completely melted. Stir in the buttermilk and vanilla extract. Set aside.
In a stand mixer with the paddle attachment, beat the softened butter on medium for 3 minutes, until fluffy. You may need to scrape down the edges once or twice. Add the sugar and beat together for another 4 minutes, until light and very fluffy. Again, scraping down the edges as needed.
Beat in the egg yolks, 1 at a time, making sure they are well incorporated before adding another yolk.
With the mixer running on medium-low, add in a third of the flour, and then half of the chocolate mixture, another third of the flour, and the rest of the chocolate, and then the rest of the flour. Mix until fully combined and the batter has formed. Stop the mixer.
In a bowl, use a hand mixer to beat the egg whites and cream of tartar until soft peaks begin to form. Gently mix in the sugar and continue to beat until soft peaks have formed.
Fold the soft peak egg whites into the batter with a large wooden spoon. Continue mixing until no more white streaks appear.
Pour the batter evenly into the prepared baking pans.
Bake for 25 to 35 minutes, or until an inserted toothpick comes out clean. Let the pans cool on a baking rack for 10 minutes. Then, gently run a sharp knife along the edge of the pan to help loosen the cakes from the sides. Carefully invert the pans. Gently tap on the bottom of the pans and lift them away from the cakes. Allow them to cool completely, about 30 minutes. Remove the parchment paper.