Boil Bonny Sweetened Condensed Milk in the closed tin for 2 hours in simmering water, to get it slightly caramelised. Remove the tin from the water and cool completely before opening.
Beat butter and 250g sugar until creamy. Beat in eggs, one at a time, until smooth. Mix flour with baking powder and stir. At this point, you may half the mixture. Add Bonny Sweetened Condensed Milk in one half portion and leave two tablespoons aside.
Chop bar chocolate into chunks and melt them in a Bain Marie. Blend 2 tablespoons of cocoa powder and the melted chocolate into the second half batter. Alternatively, pour batter into a greased (26cm diameter) baking pan. Pull batter spirally with a fork. Bake in a pre-heated oven for approx. 2 hours at 175°C.
Leave the cake in the pan for a while, remove it and cool completely on a wire rack. Whip cream and 2 tablespoons of the caramelised Bonny Sweetened Condensed Milk, and then add 2 tablespoons sugar. Spread cream on the cake and gently layer it with 1 tablespoon of cocoa powder.