Stir first 3 ingredients in large saucepan over medium-high heat until sugar dissolves and cream comes to a simmer.

Remove from heat. Add 20 ounces chocolate. Whisk until melted and smooth.

Whisk in vanilla and rum.

Transfer 1 cup warm chocolate mixture to small bowl. Whisk in remaining 2 ounces of chocolate. Cover,set aside,and let stand at room temperature. This will be the topping.

Transfer remaining chocolate mixture to a large bowl for the filling.

Chill until cold and thick. At least 6 hours.

Stir water and sugar in small saucepan over low heat just until sugar dissolves.

Remove from heat and mix in rum. Let stand.

Preheat oven to 350 degrees.

Grease and flour 9"x 9" x 2" cake pan.

Combine vanilla and coffee powder in a small bowl or cup. Swirl to dissolve coffee.

Sift flour, cocoa, and baking soda into small bowl.

Using an electric mixer, beat egg yolks and 1/2 cup sugar in medium mixing bowl until thick and light in color, about 3 minutes. Beat in coffee/vanilla mixture.

Using clean dry beaters, beat egg whites, cream of tartar, and salt in large bowl until soft peaks form.

Gradually add 1/4 cup sugar and continue beating until stiff but not dry. Fold egg whites into yolk mixture alternating with flour mixture.

Pour batter into prepared pan.

Bake for 18 minutes.

Cool cake in pan. Cake will shrink slightly.

Turn cake out onto flat surface and cut in half horizontally using serrated knife. Place bottom half on plate cut side up.

Drizzle half of syrup over layer.

Using electric mixer, beat cold chocolate filling until thick, creamy and spreadable. Spread all of filling evenly over cake layer. Filling layer will be about an inch thick.

Drizzle remaining syrup over cut side of second cake layer.

Place layer syrup side down on top of filling. Press gently.

Rewarm topping until just pourable and pour over center of cake.

Using a knife or spatula, carefully spread to edges. It's okay if a little runs over.

Cover with cake dome and chill until ready to serve.