Decadent, layered with caramel and topped with a thick chocolate icing, this cake is a little time-consuming, but well worth the effort. The key to perfect caramel is to just let the sugar and water cook to amber and never walk away during the colour stage.
- Portion size 16 servings
Grease two 9-inch (1.5 L) round cake pans; line bottoms with waxed paper. Set aside.
In small bowl, whisk boiling water with cocoa until smooth; let cool to room temperature. In large bowl, beat together butter, granulated sugar and brown sugar until fluffy, about 4 minutes. Beat in eggs, 2 at a time; beat in vanilla. Stir together flour, baking soda, baking powder and salt. Stir into butter mixture alternately with cocoa, making 3 additions of flour mixture and 2 of cocoa. Pour into prepared pans.
Bake in centre of 350°F (180°C) oven for 30 to 40 minutes or until cake layer springs back when lightly touched. Let cool in pans on rack for 20 minutes. Invert onto baking sheet, then turn right side up onto rack; let cool. Peel off paper; trim tops to level.
Caramel Filling: Follow The Caramel Method (below), boiling sugar and water for about 7 minutes or until dark amber. Remove from heat. Holding an arm's length away and averting face to avoid spatters, add hot cream; stir with long-handled spoon until blended and bubbles subside. Add butter; stir until blended. Transfer to heatproof bowl; refrigerate, uncovered and stirring twice, for about 15 minutes or until cold but not firm.
Place 1 cake layer on rack; brush top with half of the rum (if using). Spread with half of the caramel filling, leaving 1/2-inch (1 cm) border. Repeat with second layer. Refrigerate for about 1 hour or until caramel is firm but still tacky. Place 1 layer on other.
Icing: Place chocolate in bowl. In small saucepan, bring cream to boil over medium heat; pour over chocolate, whisking until melted. Let cool for 10 minutes. Pour over cake; spread over top, letting some run over edge and cover side. Refrigerate for at least 1 hour or until set. (Make-ahead: Can be refrigerated for up to 2 days; serve at room temperature.)