Instructions

1. To make dark chocolate Guinness mousse, place bittersweet chocolate and butter in a large heatproof bowl. In a medium saucepan, bring about 1-in (2.5 cm) water to a simmer over medium heat. Place bowl containing chocolate over saucepan, making sure bottom of bowl does not touch water. Stir until chocolate has melted and is smooth, about 5 minutes.
2. In a medium bowl, whisk together Guinness, salt and egg yolks until well combined. Add mixture to hot melted chocolate and using a rubber spatula stir until combined. Remove bowl from heat and set aside.
3. In a medium bowl, using a hand mixer fitted with a whisk attachment, whip egg whites until frothy. Slowly add granulated sugar and continue to whip until stiff peaks form, about 5 minutes. Set aside.
4. Clean whisk attachment for re-use. In a medium bowl, using hand mixer with whisk attachment, whip 1 cup (250 ml) whipping cream until stiff peaks form, about 4 minutes.
5. Using a rubber spatula, gently fold half of whipped cream and half the beaten egg whites into chocolate mixture until just combined. Fold remaining whipped cream and egg whites into chocolate mixture just until mixture is homogeneous. Spoon or pipe chocolate mousse equally into 8 wine or parfait glasses, taking care to fill glasses only to a maximum of three quarters full. Place glasses on a rimmed baking sheet, cover glasses with plastic wrap and refrigerate while making white chocolate mousse.
6. To make white chocolate mousse, in a small saucepan, combine ½ cup (125 ml) whipping cream and white chocolate and cook over medium heat, stirring constantly, until chocolate is melted and mixture is smooth. Transfer mixture to a medium heatproof bowl and let cool to room temperature, stirring twice, about 30 minutes.
7. Once white chocolate mixture has cooled, in a medium bowl, whip remaining ¾ cup (175 ml) whipping cream using a hand mixer fitted with whisk attachment until stiff peaks form, about 3 minutes. Using a rubber spatula, gently fold whipped cream into white chocolate mixture. Transfer to an airtight container and refrigerate alongside dark chocolate Guinness mousse for at least 3 hours or up to 24 hours.
8. When ready to serve, transfer white chocolate mousse to a piping bag fitted with a large star piping tip. Pipe white chocolate mousse over top dark chocolate Guinness mousse and garnish with chocolate shavings. Serve immediately.