Preheat oven to 350 degrees F (177 degrees C) and place the rack in the middle of the oven.  Lightly coat a 9 x 3 inch (23 x 8 cm) springform pan with melted butter or spray with a nonstick cooking spray.  Line pan with parchment paper, then lightly coat the paper with more melted butter (or spray with a nonstick cooking spray).  (Tip: Use a pastry brush to brush on the melted butter.)

Separate the eggs while still cold, placing the egg whites in one bowl and the egg yolks in another bowl.  Cover both the egg whites and yolks with plastic wrap and bring to room temperature before using (about 30 minutes).

Meanwhile, melt the butter and chocolate in the top half of a double boiler over simmering water. (Can use a stainless steel bowl over a saucepan with simmering water.)

Place egg yolks and 1/2 cup (100 grams) sugar in the bowl of your electric mixer, with the paddle attachment.  Beat on medium high speed until thick and lemon-colored, about five minutes.   (The eggs should have tripled in volume, look thick and soft, and when you lift the beater the mixture falls back into the bowl in a slow ribbon.)  Add the vanilla extract and melted chocolate mixture, beating until combined.

In a clean bowl, with the whisk attachment, beat the egg whites until foamy.  Add the cream of tartar and continue beating until soft peaks form.  Gradually add the remaining 1/2 cup (100 grams) sugar and beat until stiff peaks form.  Using a large rubber spatula or whisk, fold in a small amount of whites to the egg yolk mixture to lighten the batter.  Add the remaining egg whites, folding just until incorporated.  Do not overmix or the batter will deflate.

Pour the mixture into the prepared pan, smoothing the top.  Bake the cake for about 50 minutes to l hour or until a toothpick inserted in the center comes out with a few moist crumbs.  (During baking the surface of the cake will form a crust which will collapse when the cake is removed from the oven.)  Remove from oven and place on a wire rack to cool.  The top of the cake will have become hard with a cracked surface and lots of crumbs.  

Prepare Ganache:

For ease of handling invert cake onto a cake circle (available at cake decorating stores) or wire rack, so the bottom is now top, and refrigerate for one hour.   Place the chocolate in a stainless steel bowl.  Heat the cream and butter in a small saucepan over medium heat.  Bring to a boil.  Pour the boiling cream over the chocolate and allow to stand for 5 minutes so the chocolate can melt.  Stir with a whisk until smooth.  If desired, add the liqueur.

Assemble:

Brush any loose crumbs from the cake and place cake on a wire rack.  Put the wire rack on a baking sheet.  In this way if the chocolate ganache drips it will end up on the baking sheet, which makes clean up easier.  Using a cake spatula, cover the sides and top of the cake with about 2 tablespoons ganache.  This is called a crumb coat and seals in any cake crumbs so that your cake will have a smooth finish.  Refrigerate cake for 5 minutes to set the crumb coat.  If you have any air bubbles or crumbs in your ganache, pour glaze through a strainer.  Pour the remaining ganache into the center of the cake.  Working quickly, spread with a spatula, using big strokes to push the ganache over the sides of the cake, to create an even coating.   If there are any bare spots on sides of cake, cover with leftover ganache.   Refrigerate cake.  I find this cake is better served the next day as it allows the flavors to blend.

To serve, cut the cake with a sharp knife, wiping off the knife after slicing each piece.  If you have problems cutting the slices, warm the blade of the knife under hot running water before cutting each slice.

This cake is great served with Raspberry Pur?/font>e or Cr?e Anglaise.