This fantastic dessert should stay refrigerated until serving. Use a hot knife, wiping it clean after each cut to get pristine edges.
In large bowl, whisk together flour, sugar, cocoa powder, baking soda and salt. Whisk in coffee, oil and vanilla. Stir in vinegar. Pour into greased and parchment paper–lined 8-inch (1.2 L) round cake pan.
Bake in 350°F (180°C) oven until cake tester inserted in centre comes out clean, 40 to 45 minutes. Let cool in pan for 5 minutes; remove from pan and let cool completely on rack.
Mocha Mousse: Meanwhile, in small bowl, sprinkle gelatin over water; set aside.
In small saucepan, heat milk over medium-high heat just until 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 thick enough to thinly coat back of spoon and digital rapid read thermometer reads 160°F (71°C), about 8 minutes. Remove from heat.
Stir in gelatin mixture until melted. Whisk in bittersweet and milk chocolates, liqueur 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 9-inch (2.5 L) springform pan with parchment paper to extend 1 inch (2.5 cm) above rim. Using serrated knife, trim top of cake to level. Cut cake horizontally in half.
Place 1 layer in prepared pan. Pour in half of the mousse; spread to fill in space around cake. Top with remaining layer. Pour in remaining mousse; spread to cover top and side. Refrigerate until set, about 2 hours.
Chocolate Ganache: Place chocolate in bowl. In small saucepan, bring cream to boil; pour over chocolate and whisk until smooth. Pour over mousse; smooth with palette knife. Refrigerate until almost set, about 30 minutes. (Make-ahead: Cover and refrigerate for up to 24 hours.)
Chocolate Hearts: Meanwhile, in heatproof bowl, melt bittersweet chocolate over hot (not boiling) water. Line baking sheet with parchment paper; spread chocolate to 1/8-inch (3 mm) thickness on paper. Refrigerate until just firm, about 8 minutes.
Using 1-1/2-inch (4 cm) heart cookie cutter, cut out 8 hearts. Refrigerate until firm, about 20 minutes. Repeat with white chocolate.
Chocolate Shavings: Using vegetable peeler, shave chocolate onto waxed paper. Remove springform ring and parchment paper. Press shaved chocolate onto side of cake.
Make 16 evenly spaced 1-inch (2.5 cm) long shallow slits in top of ganache around edge of cake. Alternating white and dark chocolate hearts, insert 1 side of each heart into slit. Ingredients & Nutritional Facts Here