Preheat oven to 175°C (350°F). Grease two 8-inch or 9-inch round cake pans and line bottoms with parchment paper.
Separate 3 eggs. Set yolks aside for frosting. Let whites and yolks come to room temperature.
Sift together flour, baking soda, and salt in a bowl. Set aside.
Make buttermilk: add 2 tsp lemon juice or vinegar to 160 ml milk. Let sit 5-10 minutes.
Melt chocolate with 80 ml water in a double boiler. Stir until smooth. Cool slightly.
Beat softened butter until creamy. Add egg yolks one at a time. Gradually add sugar, beating until smooth.
Add melted chocolate to butter mixture. Beat until combined.
Alternately add buttermilk and dry ingredients (half each), mixing just until combined after each addition.
In a clean bowl, beat egg whites with a pinch of salt until soft peaks form.
Fold whipped whites into the batter gently in three additions.
Divide batter between prepared pans. Bake 30 minutes or until toothpick comes out clean. Cool in pans 15 minutes, then invert onto wire racks to cool completely.