Chocolate Swirl Marbled Blondies

Steps

1. Preheat the oven to 350°F (175°C). Lightly grease an 8-inch square pan with the canola oil and line it with parchment, leaving an overhang for lifting.
2. Melt the chocolate chips in a small heatproof bowl in the microwave in 20-second bursts, stirring between each, until smooth; set aside to cool slightly.
3. In a large bowl, whisk the melted butter with the brown sugar and the granulated sugar until glossy.
4. Whisk the eggs into the butter mixture one at a time, then whisk in the vanilla until thick and smooth.
5. In a separate bowl, whisk the flour, baking powder and salt, then fold them into the butter mixture with a spatula just until no dry streaks remain.
6. Divide the batter: scoop one-third of it into a medium bowl and leave the remaining two-thirds in the large bowl.
7. Into the one-third portion, stir the melted chocolate, the unsweetened cocoa powder and the milk until you have a smooth, dark chocolate batter.
8. Spread the plain two-thirds of the batter evenly in the prepared pan. Drop spoonfuls of the chocolate batter over the top, then drag a butter knife through both batters in loops to marble them without fully mixing.
9. Bake for 25 to 30 minutes, until the edges are golden and a toothpick inserted in the center comes out clean.
10. Cool completely in the pan on a rack, about 1 hour, then lift out by the parchment and cut into 12 bars.