Heat your oven to 350°F and grease your bundt pan really well with butter and flour.
Melt the chocolate with boiling water in a small bowl, stir until smooth, and cool for 10 minutes.
Beat butter and sugar with electric mixer until light and fluffy for about 3-4 minutes total.
Add egg yolks one at a time to the butter mix and beat well after each one.
Whisk flour, baking soda, and salt together in a separate bowl until everything's mixed evenly and well.
Add flour mix and buttermilk to butter mix, going back and forth, start and end with flour.
Stir in the cooled chocolate and vanilla extract until the batter looks smooth and well combined.
Beat egg whites in a clean bowl until stiff peaks form, then fold them gently into the batter.
Pour batter into your prepared bundt pan and bake 55-60 minutes until a toothpick comes out clean.
Cool the cake in the pan for 15 minutes, then flip it onto a cooling rack carefully.