Mocha Layer Cake With Chocolate-Rum Cream Filling

Stir first 3 ingredients in a heavy 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 rum and vanilla. Transfer 1 cup warm chocolate mixture to a small bowl; whisk the remaining 2 ounces chocolate into the mixture in the bowl and set aside for topping. Cover; let stand at room temperature. Transfer the remaining chocolate mixture to a large bowl to use.

Stir water and sugar in a small saucepan over low heat just until sugar dissolves. Remove from heat; mix in rum. Cover and let stand up to 1 day.

Preheat oven to 350 degrees F. Butter a 9- by 9- by 2-inch metal baking pan. Line the bottom with waxed paper; butter the paper.

Combine vanilla and coffee powder in a cup; swirl to dissolve the coffee. Sift flour, cocoa, and baking soda into a small bowl. Using an electric mixer, beat egg yolks and 1/2 cup sugar in a medium bowl until very thick and light in color, about 3 minutes. Beat in the vanilla-coffee mixture.

Using clean dry beaters, beat egg whites, cream of tartar, and salt in a large bowl until soft peaks form. Gradually add the remaining 1/4 cup sugar, beating until stiff but not dry. Fold 1/3 of the whites into the yolk mixture. Fold in half of the flour mixture. Fold in half of the remaining whites, then the remaining flour mixture, then the remaining whites.

Transfer batter to the prepared pan; gently spread to an even thickness. Bake cake until puffed and a tester inserted into the center comes out clean, about 18 minutes. Cool in the pan on a rack (cake may shrink slightly).

Cut around the pan to loosen the cake. Turn out onto a work surface; peel off the waxed paper. Using a long serrated knife, cut the cake horizontally in half. Place 1 half, cut-side up, on a platter. Drizzle half of the syrup (about 3 1/2 tablespoons) over it.

Using an electric mixer, beat the cold chocolate filling until thick and lighter in color, less than 1 minute. Spread all of the filling evenly over the cake layer, aligning with the sides of the cake (filling will be about 1 inch thick). Drizzle the remaining syrup over the cut side of the second cake layer. Place the layer, syrup-side down, atop the filling; press to adhere.

Rewarm the topping over low heat just until pourable. Pour the topping onto the top center of the cake. Using a small spatula, spread the topping to the edges, being careful to keep it from spilling over. Chill the cake up to 1 day, covering loosely with foil or with a cake dome after 2 hours. Cut the cake into 16 squares and serve.