INGREDIENTS:
CAKE
2 Tbsp butter 
flour or cocoa 
3 oz unsweetened chocolate 
3/4 cup unsalted butter, softened 
1/2 cup sugar 
4 eggs, separated 
pinch salt 
1/2 cup sifted all-purpose flour 
FILLING
1 1/2 cups 
heavy cream 
10 oz semisweet chocolate, chopped or broken into small chunks 
4 Tbsp dark rum 
1 tsp vanilla extract 
GLAZE
1 cup fine granulated sugar 
1/3 cup water 
7 oz semisweet chocolate, broken or chopped into small chunks

DIRECTIONS:

PROCEDURE (CAKE)
Preheat the oven to 350 deg. F.
With a pastry brush or paper towel, coat an 11x17 inch jelly-roll pan with the butter.
This will seem like a lot; be very generous. 
Sprinkle the flour or cocoa over the butter and shake the pan to coat the butter fully.
Tap the edge of the pan on a table to knock out the excess flour. 
Melt the chocolate in a double boiler or a bowl over a pan of simmering water.
Set aside to cool to lukewarm. 
Beat the egg whites with a pinch of salt until the whites cling to the beater. 
Add 1/4 cup of the sugar and beat until the whites form stiff, unwavering peaks. 
Cream the butter and the other 1/4 cup of the sugar until the mixture is light and fluffy.
Add the melted chocolate and beat in the egg yolks one at a time. 
With a rubber spatula, stir about 1/3 of the beaten eggwhites into the chocolate mixture, then pour the chocolate mixture over the rest of the whites. 
Sprinkle the flour lightly on top. Gently fold the mixture together  until no white streaks remain. 
Pour the batter into the prepared pan, spreading it evenly with a rubber spatula.
Bake in the middle of the oven for 15 to 18 minutes, or until the cake shrinks slightly away from the sides of the pan and a knife inserted in the middle comes out clean. It will still be very flat. 
Loosen the cake from the pan with a sharp knife around the sides and turn it out onto a rack to cool. (Put the rack over the pan and flip the whole thing over to keep the cake from breaking.) 

PROCEDURE (FILLING)
In a small heavy saucepan, combine the cream and chocolate and tir over medium heat until the chocolate dissolves.
Then reduce the heat to very low and simmer, stirring constantly, until the mixture thickens to the consistency of a pudding.
Pour it into a bowl and refrigerate for at least 1 hour. 
When the mixture is very cold, pour in the rum and vanilla and beat with a whisk or beater until the filling is smooth and creamy and forms soft peaks when the beater is lifted. 
Do not overbeat or you will get butter. 
Cut the cake in half to make two layers, each 8 1/2 inches wide.
Spead the filling over one layer and set the other layer on top.
Smooth out the edges with a spatula.
If one of the cake layers should break, use it on the bottom.
Refrigerate on a rack for about 1 hour. 

PROCEDURE (GLAZE)
In a small heavy saucepan, heat the sugar, water, and chocolate over medium heat, stirring constantly, until the sugar and chocolate are dissolved.
Make sure the sugar is fully dissolved or you will get a grainy glaze.
Cover the pan and let the glaze cool for about 20 minutes. 
Set a jelly-roll pan on a table with one short edge propped up.
Put the rack with the cake on something else so that it is suspended level over the pan but offset 2 inches, so that you can reach down into the lower pan with a spoon. 
Hold the pan with the glaze about 2 inches over the cake and pour the glaze on the cake.
Using a large spoon, scoop up the glaze that collects in the jelly-roll pan and put it back on the cake.
Keep doing this until the glaze begins to stop flowing smoothly.
You should end up with a thick, even layer of glaze on the cake. 
Refrigerate the cake until the glaze is firm, 10 to 20 minutes. 
Serve by cutting into 35 small equal pieces, 5 in each row across and 7 in each row down.
For cutting, use a sharp knife that has been dipped in warm water and wiped off between slices. 
Keep refrigerated, but for maximum flavor, allow to come to room temperature before serving.