Preheat the oven to 350F. Spray an 8x8-inch square baking pan with baking spray and line with parchment paper.
In a small saucepan, melt the butter over medium heat. Remove from the heat and whisk in 1/2 cup chocolate chips. Continue whisking until fully melted. Transfer to a large bowl and let cool for a few minutes. (You can also melt the butter in a large bowl in the microwave, then stir in the chocolate until melted.)
In another bowl, sift together the flour and cocoa powder. Whisk in the salt.
To the melted chocolate, whisk in the sugars, eggs, and vanilla until smooth. Fold in the flour mixture until well combined. Fold in the remaining 1/2 cup chocolate chips. Pour the batter into the prepared pan and use a spatula to spread the batter to the edges of the pan.
Drop the peanut butter over the top of the batter by teaspoonfuls. Use the tip of a knife or a screwer to swirl it into the batter.
Bake for 40 to 45 minutes or until the center doesn’t wobble when the pan is gently shaken, or 35 minutes for a super fudgy center. Cover the brownies with foil after 25 minutes if the peanut butter appears to be browning. (The bake time may increase by 5 to 10 minutes if covered.) Let the brownies cool in the pan completely before slicing.
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Notes
Room temperature eggs will mix more smoothly into the batter than cold eggs. Bring the eggs out of the fridge ahead of time or place them in a bowl of warm water to quickly warm them to room temperature.
I highly recommend using a kitchen scale to measure your flour as it’s the most accurate method. However, if you don’t have a scale, be sure to fluff your flour with a spoon and then spoon it into your measuring cup before leveling it off with a knife. This method is the best way to measure flour without overpacking the measuring cup.
Avoid over-mixing the brownie batter as it’ll lead to a batch of tough brownies. Skip with mixing the batter with a spatula and skip using a mixer.
For beautiful clean cuts, wipe the knife on a damp cloth or run it under warm water between cutting.
When melting the chocolate, make sure not to get water in the pan as it can cause the chocolate to seize.
Don’t skip sifting the dry ingredients. It helps the ingredients mix together, aerate the dry ingredients, and breaks up any lumps. This helps prevent dense or overmixed brownies.
When swirling the peanut butter, less is more. If you swirl too much, you will mix the peanut butter into the batter.