Preheat the oven to 175 C. Grease and line an 8x8 baking dish and set aside. (Lining the dish with baking paper helps with easy removal and slicing)
In a bowl placed over a pot of simmering water, melt the butter, chocolate and cocoa powder, stirring occasionally until you have a smooth mixture. Set aside to cool.
Beat the eggs, sugar and vanilla in a large bowl till pale and frothy, about 3 minutes.
Add the cooled chocolate mixture to the egg mixture and beat again till combined.
Fold in the flour and salt (if using) till just combined, do not over-mix.
Pour into the prepared dish and even the top.
Drop spoonfuls of the melted peanut butter on top of the batter, alternating with spoonfuls of the jam. Using a sharp knife or toothpick, swirl them around.
Bake for 30 to 35 minutes, until a toothpick poked in the center comes out with only moist crumbs, no wet batter. Over-baking the brownies till the toothpick is completely clean will make them dry.
Preferably allow to cool completely before slicing and devouring!