Preheat the oven to 350°F and bring all ingredients to room temperature. Grease your cake pan.
Whisk together thoroughly cake flour, baking powder, and salt.
Over hot water, melt unsweetened chocolate and add boiling water.
In a large bowl, beat butter at a high speed until creamy.
Add sugar to the bowl and beat until light and fluffy, about five minutes.
Beat in egg yolks, one at a time.
Add the cooled chocolate mixture.
Beating at a low speed, add the flour mixture to the butter mixture in three parts, alternating with the milk. Beat batter until smooth after each addition. Add vanilla.
Using clean beaters, in a large bowl beat egg whites at a medium-high speed until they are stiff but not dry. Fold them lightly into the batter.
Scrape batter into a prepared pan and bake until a cake tester inserted into the center comes out clean, approximately 30 minutes. Let cool before frosting.