Heat 1/2 cup heavy cream until steaming but not boiling.
Pour hot cream over dark chocolate and butter in a heatproof bowl and let sit for 1-2 minutes.
Stir mixture until smooth and glossy, then allow to cool slightly to thicken.
In a chilled mixing bowl, whip cold heavy cream until soft peaks form.
Add powdered sugar, espresso mixture, and vanilla extract and continue whipping until medium peaks form.
Layer chocolate base, coffee mousse, and chocolate crumbs in clear glasses, repeating layers if space allows.
Finish with a final dollop of mousse and sprinkle of crumbs.
Garnish each trifle with raspberries, blueberries, mint sprigs, and optional chocolate shavings.
Chill trifles for at least 1 hour before serving.