Preheat oven to 350°F (175°C). Grease and line two 9” cake pans.
Melt chocolate with boiling water. Set aside to cool.
Whisk flour, baking soda, and salt.
Cream butter and sugar until fluffy. Add yolks, vanilla, and cooled chocolate.
Alternate adding dry ingredients and buttermilk. Mix gently.
Fold in whipped egg whites.
Weigh total batter and divide evenly: **750 grams per pan**.
Bake 30–35 minutes or until a toothpick comes out clean. Cool before frosting.