Whisk together eggs and sugar in a heatproof bowl set over (but not in) a pot of simmering water until doubled in volume and a thermometer registers 160 degrees (6 to 8 minutes). Remove from heat. Add chocolate, cocoa, espresso powder, and salt. Stir in butter until melted. Let cool slightly.

Meanwhile, whip cream to stiff peaks. Stir one-third of whipped cream into chocolate mixture until lightened, then fold in remaining whipped cream. Divide half of mousse between two glasses; sprinkle with crushed cookies. Top with remaining mousse and refrigerate until set, at least 1 hour and up to 1 day. To serve, top with lightly whipped cream and more cookie crumbs.