Peanut Butter Swirl Walnut Brownies

Steps

1. Preheat the oven to 350°F (175°C). Brush an 8-inch square pan with the canola oil and line it with parchment, leaving an overhang.
2. Spread the walnuts on a small tray and toast in the oven for 6 to 7 minutes until fragrant; let cool.
3. In a heatproof bowl set over barely simmering water, melt the butter and chocolate chips together, stirring until smooth. Remove from the heat.
4. Whisk the unsweetened cocoa powder into the warm melted chocolate until smooth.
5. Whisk the granulated sugar and the brown sugar into the chocolate mixture.
6. Whisk in the eggs one at a time, then the vanilla and salt, until the batter is glossy.
7. Fold in the flour just until no dry streaks remain, then fold in the toasted walnuts. Spread the batter evenly in the pan.
8. In a small bowl, stir the peanut butter with the powdered sugar until smooth; warm it for 10 seconds in the microwave if stiff so it drops easily.
9. Dollop the peanut butter mixture over the batter in spoonfuls, then drag a butter knife through to make swirls.
10. Bake for 25 to 30 minutes, until the edges are set and a toothpick inserted 1 inch from the edge comes out clean or with a few moist crumbs; the center will firm as it cools.
11. Cool in the pan on a rack for at least 1 hour, then lift out by the parchment and cut into 12 bars.