Prepare an 8-inch (20 cm) springform pan or a cake ring wrapped with aluminum foil forming its bottom. Using softened butter, grease the bottom and sides of the pan or ring and line it with parchment paper. Place the prepared cake ring on a baking sheet.

To make the cake batter, chop the chocolate and cut the butter into small cubes. Melt chocolate and butter in a double boiler or bain marie, stirring occasionally with a wooden spoon. Alternatively, use the microwave with 30-second increments or the stovetop with low heat. Let the mixture cool for 5 minutes.

Place salted butter caramel in a heat-prove small bowl, warm it up in the microwave, and add it to the melted chocolate mixture.

Place sugar and eggs in a large mixing bowl and whisk with an electric mixer for 5 minutes. The mixture will double in volume and look foamy. Pour the cooled melted chocolate and butter mixture into the eggs and sugar preparation and whisk on low speed. Add the sifted flour and gently mix with a rubber spatula.

Pour the batter into the prepared mold and let it rest for 2 hours at room temperature.

Heat the oven to 410°F (210°). Bring the mold to the middle rack of the preheated oven and bake for 5 minutes. Then lower the oven temperature to 250°F (120°C) and bake for another 40 minutes. Watch out for creating a meringue crust on top of the cake and a slightly wobbly center.

Remove the cake from the oven and let it cool to room temperature. Then refrigerate it for at least 4 hours, better overnight.

Unmold the cake and let it stay at room temperature for 2-3 hours before serving to regain its melting texture. To remove the cake from the cake ring, pass a thin knife between the parchment paper and the ring to loosen the cake from the edges.

For slicing, use a long sharp knife dipped in hot water, dying it out between slices.