Fill a small pot with a few inches of water and bring to a low simmer over medium-high heat. Reduce heat to keep at a low simmer. Place chocolate in a heat-safe bowl that will fit on top of pot and place over simmering water. Melt chocolate, stirring occasionally until smooth. Remove from heat and allow to cool 5 minutes.
Place whipping cream in a large bowl and use an electric mixer to beat on high speed until stiff peaks form. Transfer to a smaller bowl and set aside.
Clean bowl and beaters and wipe dry. Place egg whites in bowl and use an electric mixer to beat on high speed until egg whites reaches stiff peaks.
Whisk egg yolks, honey, vanilla, espresso, and Guinness in a small bowl until smooth. Whisk into slightly-warm melted chocolate until smooth.
Very gently fold half of whipped cream and half of beaten egg white into chocolate mixture, folding until just combined. Fold in remaining whipped cream and egg white until just combined and no streaks of chocolate remain.
Spoon mousse into four serving dishes and refrigerate 2 hours before serving. When ready to serve, garnish tops with whipped cream and chocolate shavings.