Melt your chocolate in a heatproof bowl in 15-second increments until just melted. Do not overheat or you could burn your chocolate. Add your warm water to the chocolate and whisk to combine.
Place egg whites in a bowl with the whisk attachment and whip until you reach firm but moist peaks. They shouldn't look broken or crumbly. Set aside.
Combine buttermilk and vegetable oil and set aside
Whisk together flour, baking soda and salt and set aside.
Cream together the butter and sugar until light and fluffy in color
Add in your egg yolk while mixing on low, letting it mix in fully before adding in your last egg yolk.
While mixing on low, add in your melted and cooled chocolate and vanilla and mix until combined.
Mixing on low, add in 1/3 of flour mixture, then 1/3 of buttermilk. Repeat two more times with remaining flour and buttermilk.
Gently fold in your egg whites until blended.
Pour your batter into your prepared pans and bake immediately for 30 minutes or until a toothpick comes out from the center clean. Let cool before frosting.