Line the bottom of a square springform pan (24 x 24 cm) with baking paper. Melt 75 g butter in a wide saucepan. Add 350 g sugar. Caramelise at low to medium heat while stirring until light brown.
Deglaze with 600 g whipped cream. Bring everything to the boil. Simmer for about 1 1⁄4 hours until the mixture is thick and creamy, stirring frequently (stir constantly at the end).
Divide 150 g dark chocolate into pieces and melt in a hot water bath. Leave to cool for about 5 minutes. Stir into the caramel mass and bring to the boil briefly. Pour the mixture into the mould. Let it set overnight.
Remove the caramel from the pan, turn it over on a board, remove the baking paper. Break into pieces with a parmesan knife, for example. Store in a cool, dry and airtight place.