Chocolate Mousse Cake

Grease 9-inch (2.5 L) springform pan; line bottom with parchment or waxed paper. Set aside.

In large bowl, beat butter with sugar until fluffy; beat in eggs, 1 at a time. Beat in vanilla. In separate bowl, sift together flour, cocoa, baking powder, baking soda and salt. Stir into butter mixture alternately with buttermilk, making 3 additions of dry ingredients and 2 of buttermilk. Scrape into prepared pan; smooth top.

Bake in centre of 350°F (180°C) oven until cake tester comes out clean, 30 minutes. Let cool on rack for 20 minutes. Remove side of pan; let cool. (Make-ahead: Cover and set aside for up to 24 hours.)

Chocolate Mousse: Meanwhile, in small bowl, sprinkle gelatin over 2 tbsp (25 mL) water; set aside. In small saucepan, heat milk over medium-high heat just until tiny bubbles form around edge. In heatproof bowl, whisk together egg yolks, sugar and salt ; slowly whisk in milk. Place bowl over saucepan of simmering water; cook, stirring, until custard is 160°F (70°C) and thick enough to coat back of spoon, about 8 minutes. Remove from heat.

Stir in gelatin mixture until melted. Whisk in bittersweet and milk chocolates, rum (if using) and vanilla. Refrigerate, stirring twice, until cold and mixture can mound on spoon, about 15 minutes.

In separate bowl, whip cream; whisk one-quarter into chocolate mixture. Fold in remaining cream.

Line side of 10-inch (3 L) springform pan with parchment paper to extend 1 inch (2.5 cm) above rim. Cut cake horizontally in half. Place 1 layer in prepared pan. Pour in half of the mousse, letting mousse extend over side of cake. Top with remaining layer. Pour in remaining mousse to cover top and side of cake; smooth top. Refrigerate until set, 2 hours.

Chocolate Ganache: Place chocolate in bowl. In small saucepan, bring cream to boil; pour over chocolate and whisk until smooth. Let cool for 30 minutes. Pour over mousse; smooth with palette knife. Refrigerate until set, about 1 hour. (Make-ahead: Cover and refrigerate for up to 24 hours.)