What is peanut butter swirl?

A technique for adding a contrasting ingredient (peanut butter, Nutella, dulce de leche, jam, even brownie batter of a different color) on top of a base batter and dragging a knife or skewer through to create marbled patterns visible in the finished bake. Unlike folding in, which distributes the ingredient evenly, swirling creates discrete pockets and visual contrast. For brownies, the standard technique: spread the brownie batter in the pan, dollop 6-8 spots of room-temperature peanut butter on top, then drag the tip of a butter knife through the dollops in a figure-8 pattern. Bake normally — the peanut butter stays distinct from the brownie crumb because the two have different set temperatures.