A chocolate-banana batter is swirled with peanut butter for a decadent, yet not too sweet, brownie. Loosely adapted from Sweet, by Yotam Ottolenghi and Helen Goh.
Preheat the oven to 400 F. Line a 9-inch square baking pan with baking parchment.
In a heavy-bottomed saucepan over low heat, melt the butter together with the chopped semisweet and unsweetened chocolate, stirring occasionally. As soon as almost all the chocolate and butter is melted, turn off the heat and stir to help melt the last few bits.
Allow the melted butter and chocolate to cool to just warm, and then mix in the mashed banana.
Meanwhile, sift together the flour, cocoa, and salt.
In a large bowl, use an electric mixer to whisk the eggs with the sugar until pale and creamy, about 3 minutes. Add the chocolate-butter-banana mixture and fold it in with a rubber or silicone spatula, being careful not to overmix.
Fold in the sifted flour, cocoa, and salt, just until fully incorporated.
Scrape the brownie batter into the parchment-lined baking pan.
In the microwave or a small saucepan, gently heat the peanut butter until it softens to an almost liquid consistency. (30 seconds in the microwave at 40% power should do the trick.)
Spoon dollops of the peanut butter on top of the brownie batter. Then run a skewer or toothpick through the batter to create a marbled effect.
Place the pan in the preheated oven and bake for 23-25 minutes, until there is just a slight wobble in the center.
If you can restrain yourself, cool completely before slicing.
For the peanut butter, I used Simply Jif, which is either barely sweetened or unsweetened, and relatively low sodium, but creamier than the "natural" style peanut butters. However, feel free to use a sweeter peanut butter, or to try a natural peanut butter. (If you do, let me know how it turns out!)
When I made these the second time, I used about 90 grams of peanut butter--if you use that amount, the brownies will be . . . very peanut butter forward. I liked that, but if you want a slightly less peanut-butter-heavy result, use 75 grams.
These are very fudgy brownies--even after they are fully cooled, the brownies toward the middle of the pan will be very gooey. Most people think that's a good thing.