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1
For filling and topping: Stir first 3 ingredients in heavy large saucepan over medium-high heat until sugar dissolves and cream comes to simmer.
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2
Remove from heat.
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3
Add 20 ounces chocolate; whisk until melted and smooth.
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4
Whisk in rum and vanilla.
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5
Transfer 1 cup warm chocolate mixture to small bowl; whisk remaining 2 ounces chocolate into mixture in bowl and set aside for topping.
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6
Cover; let stand at room temperature.
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7
Transfer remaining chocolate mixture to large bowl to use as filling; chill until cold and thick, at least 6 hours and up to 1 day.
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8
For syrup: Stir water and sugar in small saucepan over low heat just until sugar dissolves.
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9
Remove from heat; mix in rum.
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10
Cover and let stand up to 1 day.
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11
For cake: Preheat oven to 350u00b0F.
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12
Butter 9x9x2-inch metal baking pan.
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13
Line bottom with waxed paper; butter paper.
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14
Combine vanilla and coffee powder in cup; swirl to dissolve coffee.
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15
Sift flour, cocoa, and baking soda into small bowl.
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16
Using electric mixer, beat egg yolks and 1/2 cup sugar in medium bowl until very thick and light in color, about 3 minutes.
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17
Beat in vanilla-coffee mixture.
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18
Using clean dry beaters, beat egg whites, cream of tartar, and salt in large bowl until soft peaks form.
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19
Gradually add remaining 1/4 cup sugar, beating until stiff but not dry.
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20
Fold 1/3 of whites into yolk mixture.
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21
Fold in half of flour mixture.
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22
Fold in half of remaining whites, then remaining flour mixture, then remaining whites.
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23
Transfer batter to prepared pan; gently spread to even thickness.
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24
Bake cake until puffed and tester inserted into center comes out clean, about 18 minutes.
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25
Cool in pan on rack (cake may shrink slightly).
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26
Cut around pan to loosen cake.
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27
Turn out onto work surface; peel off paper.
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28
Using long serrated knife, cut cake horizontally in half.
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29
Place 1 half, cut side up, on platter.
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30
Drizzle half of syrup (about 3 1/2 Tablespoons) over cake.
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31
Using electric mixer, beat cold chocolate filling until thick and lighter in color, less than 1 minute.
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32
Spread all of filling evenly over cake layer, aligning with sides of cake (filling will be about 1 inch thick).
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33
Drizzle remaining syrup over cut side of second cake layer.
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34
Place second layer, syrup side down, atop filling; press to adhere.
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35
Rewarm topping over low heat just until pourable.
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36
Pour topping onto top center of cake.
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37
Using small spatula, spread topping to edges, being careful to keep it from spilling over.
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38
Chill cake up to 1 day, covering loosely with foil or with cake dome after 2 hours of chilling.
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39
Cut cake into 16 squares and serve.
