In a medium saucepan, combine 2 cups cream, the chocolate, half of the toffee bars, the espresso powder and salt. Cook over medium heat, whisking occasionally, until melted and smooth. Pour the mixture into a medium-large bowl and refrigerate, stirring occasionally, until cold, about 3 hours.
Pour the remaining 1/2 cup cream into a medium bowl. Stir in the confectioners sugar and vanilla and beat until stiff but not dry. In a large bowl, whip the cold chocolate mixture until thick and creamy.
Spoon or pipe the mousse into glasses, mugs or small bowls. Dollop with the whipped cream and top with the remaining toffee bars.