Preheat oven to 356°F (180°C). Grease a 9 inch bundt pan or 8-inch round cake pan thoroughly with butter, then dust with flour, tapping out excess.
Clean your mixing bowl with lemon juice or vinegar to remove any grease. Add egg whites and salt, then whip with an electric mixer until stiff, glossy peaks form (3-4 minutes). Refrigerate until needed.
6 egg whites, lemon juice or vinegar
In a large bowl, cream together room-temperature butter and sugar until light and fluffy. Beat in egg yolks one at a time, then add milk.
1 pinch salt
In a separate bowl whisk the butter, sugar, egg yolks and milk until creamy.
Create a double boiler by placing a heatproof bowl over simmering water. Melt chopped chocolate, stirring occasionally until smooth. Remove from heat and let cool slightly.
5 oz baking semi dark chocolate
Pour batter into prepared pan and bake for 45-50 minutes, until a toothpick inserted in center comes out with just a few moist crumbs.