Boil the tin of condensed milk in a saucepan of water for four hours to make dulce de leche.
Preheat the oven to 175°C and grease a 20cm square cake tin.
Melt the chocolate and butter in a bowl suspended over simmering water.
Whisk in cocoa powder, eggs, and sugar, then fold in the flour.
Pour half the batter into the tin and dollop dulce de leche on top.
Swirl the dulce de leche with a spoon to create a marbled effect.
Add the remaining batter and more dulce de leche, سپس marble again.
Bake for 30-40 minutes until a skewer comes out clean.
Cool the cake before cutting into 8cm to 10cm squares.
Serve with chocolate sauce and cream.