Step 1

Preheat the oven to 350°F and line an 8×8 square baking pan with parchment paper, leaving parchment paper hanging over the sides of the pan so that you can easily remove the brownie blondies from the pan later.

Step 2

Melt the butter in the microwave or in a small pan over the stove. Transfer the melted butter to a medium sized bowl.

Step 3

Add in the light brown sugar and granulated sugar. Whisk the butter and sugars together. Keep whisking until the mixture resembles a paste that pulls away cleanly from the side of the bowl when you mix it.

Step 4

Add the eggs and vanilla and whisk well.

Step 5

Pour the flour, cocoa powder, and salt into the bowl and mix with the whisk until just combined. Do not over-mix! Set aside while you make the blondies.

Step 6

In another medium bowl, add melted butter, brown sugar, and granulated sugar. Again, whisk the butter and sugars together until it resembles a paste that pulls away cleanly from the side of the bowl.

Step 7

Add in the eggs and vanilla, then whisk well.

Step 8

Add the flour and salt into the bowl and mix with the whisk until just combined. Again, do not over-mix!

Step 9

Use a measuring cup to scoop about 1/4-1/3 cup of brownie batter into one corner of the baking pan. Use a different measuring cup to scoop the 1/4-1/3 cup of blondie batter into the space beside it. They will start to spread together since the batter is runny - that's okay.

Step 10

Add another scoop of brownie batter to the corner opposite where the other scoop of brownie batter was, then if there is room, do the same with the blondie batter. Add a scoop of brownie batter over top of the blondie batter, then a scoop of blondie batter over top of one of the brownie batter areas, and alternate this way until all batter is gone. (Process photos are in the post above for reference).

Step 11

Take a butter knife and lightly draw an "S" shape in the top of the batter with the tip of the knife, creating a marble effect (Photo in post above for reference). This step is optional, as it just creates a prettier look on top. Bake for 25-35 minutes or until a toothpick inserted in the middle of the brownie blondies comes out with just a few wet crumbs on it (but not covered in batter).

Step 12

Let cool completely in the pan before slicing!