Preheat oven to 350°F Line a 9-inch spring form cake pan with a circle of parchment paper.
Add whipping cream into a saucepan and gently heat, when the cream is fairly hot add butter and chocolate chips and gently whisk until the chocolate is melted. Remove from heat and add rum or vanilla. Cool to room temperature.
In a large bowl add eggs, sugar and sea salt and beat with an electric mixer until the eggs become fluffy. Slowly add the cream mixture and beat until combined.
Pour the batter into the cake pan and place it on a baking pan. Bake for one hour. The cake may look a bit soft in the middle this is fine. Place the cake on a wire rack and cool to room temperature, then cover and chill for 4 hours or overnight.
Loosen the sides before releasing the clasp, remove the ring and place on a cake plate. Dust with cacao and garnish with berries.