Heat your oven to 150°C Fan/170°C/Gas mark 3/340°F
Grease your tin if using a brownie tin, or line your tin if using a normal tin.
Melt your butter slowly over a low heat.
Mix in your chocolate, stirring constantly until it has fully melted.
Leave to cool.
Make your peanut butter spread by mashing your banana and then mixing the banana and all of the other spread ingredients together.
Add your eggs, sugars, vanilla extract to your mixer bowl and beat for 10-20 seconds until the mixture has little air bubbles on the top.
Add your egg mixture to your melted and cooled chocolate mixture and fold together, then return the whole lot to your mixer bowl.
Mix your flour and salt together, just with a fork or spoon, and add to your brownie batter.
Mash your banana and add to your brownie mixture.
Beat for about 10 seconds on medium-high, then stop and scrape down the sides and base of your bowl with your spatula to ensure everything is mixed in. Beat for another 10 seconds or so on medium high.
Take a desertspoonful of brownie batter and add to your brownie tin, or if using a normal tin spread half of the mixture over the base.
Add about half a teaspoonful of your peanut butter mixture onto the top of your brownie mixture and spread out with your skewer or cocktail stick. If using a normal tin, dot the spread in rows of 4x4 evenly over the brownie mixture.
Cover with the remaining brownie mixture and spread out so that the filling is hidden.
Bake for 20 minutes if using a brownie tin or 25 minutes for a normal tin. The centre should still look a little gooey when you take the brownies out of the oven, or a skewer poked into the centre should come out with a few bits of fudgy crumbs clinging to it.
Leave to cool in the tin for 5 minutes then remove to a cooling rack and allow to fully cool.