Line a baking tray with baking paper (I used a 25x25cm tin).
In a saucepan on a very low heat, melt the butter.
When it's starting to melt, add in the dark chocolate and milk chocolate and the sugar and stir until it's all melted together.
Once they are melted remove from the heat and leave to cool for 5-10 minutes.
Beat in each egg, one by one.
Stir in the flour.
Pour into your baking tin and top with the peanut butters (SEE NOTE 3).
Using a knife, swirl the mixture so that some of the batter is on top of the peanut butter and some remains on top, making a pretty pattern.
Bake for 30 minutes, or until the edges are just solid but the middle still has a little wobble. Don't overbake them!
Ovens do vary so don't be alarmed if you recipe takes that little bit longer. Brownies are better under-done than over.
Allow to cool COMPLETELY before cutting up (if you can bare it!)
Note 1 - ButterPlease do use butter rather than margarine, the taste and texture will be much better. If you have a dairy allergy or similar and need to substitute, it will work (obviously you'll need to change up the chocolate too) but you might need to adjust the cooking time a little depending on the consistency of the spread you use.Note 2 - ChocolateThis recipe uses dark chocolate and milk. I wouldn't recommend only using milk chocolate as they just won't be dense/fudgy in the same way. A slightly difference ratio if you don't have enough of either will be fine though.Note 3 - Peanut ButterI like a mixture of crunchy and smooth (half and half) peanut butter for this. (I just mix them together in a small bowl before using.) But if you don't have both types, either one will be fine. I like Pip&Nut brand peanut butter. It has quite a runny texture. If the peanut butter you are using does NOT have a runny texture, you can pop it into the microwave for 30 seconds or so before using to make it more runny. It it's too stiff it won't 'swirl' through the brownie batter.