Preheat the oven to 350 degrees F. Line a baking tray with parchment paper and set an 8 by 2 1/2-inch ring on it, Wrap an 8-inch cardboard round with foil and set aside.
In a small bowl, sift together the flour and the baking soda. Set aside. In a double boiler or a metal bowl placed over simmering water, melt the chocolate. Keep warm.
Meanwhile with the paddle of an electric mixer, cream the butter until light. Gradually add 1 cup sugar and continue to cream until fluffy. (It is very important that this mixture is light and fluffy.)
Beat in the egg yolks, 1 at a time, the whiskey, and the vanilla. Stop the machine and scrape in the melted chocolate. Continue to mix until well combined. Remove the bowl from the machine and fold in half the flour mixture. Fold in the remaining flour.
With a clean whisk and bowl, whip the egg whites until soft peaks form. Gradually add the remaining 1/4 cup sugar and continue to whisk until shiny and firm, but not stiff. Stir 1/4 of the whites into the batter to lighten, then fold in the remaining whites. Pour the batter into the prepared cake ring and bake for 1 hour. Invert immediately onto the foil-covered cardboard round and run a sharp knife around the sides of the cake, loosening the cake from the ring. Do not remove the ring from the cake. Let cool completely on a rack. Then, carefully lift off the ring.
Dust the cake with sifted powdered sugar and serve with ice cream or whipped cream.