Preheat the oven to 180 C / Gas 4. Butter a 1L bundt pan.
In a saucepan over a low heat melt the butter and chopped dark chocolate, remove from heat when almost melted. Set aside.
In a small bowl, combine flour and salt. Set aside.
In a separate bowl mix the sugar, eggs and vanilla until pale yellow and fluffy, about 10 minutes. Alternately add chocolate mixture and flour mixture to the egg batter, beginning and ending with the chocolate. Mix until just incorporated.
Pour half of the cake mixture into the prepared pan, break caramel chocolate into squares and layer side by side around the center of the cake mixture, sprinkle the toasted nuts over the chocolate squares.
Pour the rest of the cake mixture into the pan, spreading it evenly over the caramel chocolate and nuts.
Bake the cake in the preheated oven for 45 to 55 minutes. Transfer to a wire rack to cool for about 20 minutes. Invert onto serving plate. Enjoy.