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 heat and whisk in 1/2 cup of chocolate chips until fully melted. Transfer to a large bowl and let cool for a few minutes.

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 of chocolate chips. Pour the batter into the prepared pan and use a spatula to spread it to the edges.

Drop the peanut butter over the top of the batter by teaspoonfuls. Use the tip of a knife to swirl it into the batter.

Bake for 40 to 45 minutes or until the center doesn’t wobble when shaken, or 35 minutes for a super fudgy center. Cover with foil after 25 minutes if peanut butter looks like it's browning. Let cool completely in the pan before slicing.