Preheat and prep: lightly grease a 9" x 13" metal pan, line it with a parchment sling, then grease the parchment paper as well. Preheat oven to 350°F.*1
Whisk together most blondie ingredients: combine melted butter, brown sugar, sugar, vanilla, and eggs and mix by hand or with a mixer on low until everything is fully incorporated into a smooth and creamy mixture.
Add flour: blend together flour, baking powder, and salt in a small bowl. Blend the dry mixture into the batter by hand until fully incorporated.
Transfer to pan and level: transfer batter to prepared pan, and spread into an even layer all the way to the edges. Place pan in fridge to chill while you prep the brownie batter.*2
TO MAKE THE BROWNIE BATTER:
Melt chocolate and butter: over a double boiler or in the microwave, melt butter and dark chocolate chips together, mixing until fully combined, shiny, and smooth.*3 Allow chocolate to cool for 5-10 minutes (really hot chocolate could cook our eggs in the next step).
Combine sugar and eggs: whisk together sugar, eggs, and vanilla until smooth.
Add chocolate: add slightly cooled chocolate mixture to the egg mixture and mix until fully combined.
Add flour: blend together flour, baking powder, and salt in a small bowl. Blend the dry mixture into the chocolate batter by hand just until fully incorporated.
Pour into pan: remove the blondie pan from the fridge.*4 Carefully drop brownie batter in a bunch of large spoonfuls spread evenly across the top of the chilled blondie batter. Use a spatula to gently spread the brownie batter into a solid layer until it fully covers the blondies. (These two batters are a little soft and may mix up a little, in which case, don’t worry! They’ll still taste just as delicious.)
Bake: bake for 36-42 minutes, until the surface of the bars looks dry and fully set, nothing moves when you gently shake the pan, and a toothpick inserted into the center comes out clean.
Cool to room temperature: cool bars fully before using the parchment sling to transfer the bars to a cutting board to slice (or simply slice them right in the pan). Pass out a couple of bars to share some sweetness, and enjoy this scrumptious chocolate + vanilla mash-up!
Notes
*1Can you make these bars in an 8” square pan? While technically you can make a half batch and significantly reduce the baking time for an 8" square pan, I found the texture and look was much nicer in a 9" x 13" pan, so I'd recommend sticking with the full batch size for best results.
*2Save some dishes! After finishing the blondie batter, just give your tools a quick rinse (no need to sanitize) as you can use that same bowl and those tools to mix the brownie batter instead of dirtying new ones.
*3Melting Chocolate: if using the microwave to melt your chocolate, set it to 50% or lower power and heat for 30-second intervals, stirring well between each round, to prevent the chocolate from burning.
*4Make sure blondies are fully chilled: if you're really speedy mixing up your brownie batter, and notice your blondies are still pretty soft, just re-chill them in the fridge, or even the freezer for another 10-15 minutes before topping with the brownie batter.
Storage: store air-tight at room temperature for up to a week.You can freeze baked bars, but they’ll lose some of their fudginess in the thaw and are best enjoyed fresh.
It’d mean so much to me if you could please give the recipe card a ⭐ rating on the site and leave a comment to help others find this sweet recipe! Xo, Jocelyn @ Mint+Mallow