Prepare for Baking:

1. Preheat and prep: Preheat your oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving some overhang on two sides for easy removal. Alternatively, grease the pan well with butter or cooking spray. Parchment paper is highly recommended for easiest removal and cleanup.

Make the Batter:

2. Melt the butter: In a large microwave-safe bowl, melt the butter in the microwave (about 45-60 seconds). Let it cool for a few minutes until it’s warm but not hot—you should be able to comfortably touch the bowl. If the butter is too hot, it will cook the eggs in the next step.
3. Mix butter and sugar: Add the brown sugar to the melted butter. Stir vigorously with a wooden spoon or whisk for about 1 minute until the mixture is smooth, glossy, and well combined. The sugar won’t dissolve completely—that’s normal and expected. The mixture should look like thick, wet sand.
4. Add eggs and vanilla: Crack the eggs into a small bowl first to check for shells, then add them one at a time to the butter-sugar mixture, stirring well after each addition. The batter will look separated and slightly curdled at first—keep stirring and it will come together. Add the vanilla extract and mix until the batter is smooth and glossy. Beat vigorously for about 30 seconds to incorporate some air, which helps create that beautiful crackly top.
5. Add dry ingredients: In a separate medium bowl, whisk together the flour, baking powder, and salt. This ensures even distribution of the leavening and salt. Pour the dry ingredients over the wet ingredients and stir gently with a rubber spatula or wooden spoon just until no flour streaks remain. Don’t overmix—stop as soon as you don’t see any dry flour. Overmixing develops gluten and makes the bars tough instead of tender.
6. Fold in chips: Add the chocolate chips (and nuts if using) to the batter. Fold them in gently with a spatula until evenly distributed throughout the batter. Reserve a handful of chocolate chips to sprinkle on top if desired for a prettier presentation.

Bake:

7. Spread in pan: Scrape the batter into the prepared pan. It will be quite thick and sticky—this is correct! Use a spatula to spread it evenly to all corners of the pan, smoothing the top. The batter should be about 1 inch thick. If you reserved some chips, sprinkle them evenly over the top now.
8. Bake the bars: Bake in the preheated oven for 25-30 minutes. The bars are done when the top is golden brown with a beautiful crackly surface, the edges are set and slightly pulling away from the pan, and a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter, but definitely not completely clean). The center should still look slightly soft—it will continue to firm up as it cools. Don’t overbake or the bars will be dry instead of chewy and fudgy.

Cool and Cut:

9. Cool completely: Remove the pan from the oven and place it on a wire cooling rack. Let the bars cool completely in the pan—this takes at least 1 hour. Congo Bars need to cool fully to set up properly and achieve their signature chewy texture. Cutting them warm will result in a gooey mess.
10. Cut into bars: Once completely cool, use the parchment paper overhang to lift the entire slab out of the pan onto a cutting board. Using a sharp knife (wipe it clean between cuts for neatest bars), cut into 24 squares (6 rows by 4 rows). For extra clean cuts, refrigerate the bars for 30 minutes before cutting.