Fudgy chocolate cake with leftover easter eggs

220 g unsalted butter 100 g granulated sugar 100 g brown sugar 3 eggs 1 vanilla pod or 2 tsp vanilla extract 1/4 tsp salt 120 g all-purpose flour 70 g cocoa powder 270 g chocolate (leftover Easter eggs or any chocolate) ½ tsp flaky sea salt (for finishing)