Preheat oven to 350°F (177°C). Line an 8 x 4 inch baking pan with parchment paper.
In a large mixing bowl, add peanut butter and egg whites. Use a whisk to mix until egg whites are fully incorporated into the peanut butter. As you are mixing, the peanut butter will get thick and chunky and may get stuck on your whisk. Make sure to release the peanut butter stuck on your whisk (I just tap the whisk a few times against the edge of the bowl), and keep whisking until batter is smooth with no egg streaks remaining.
Add in powdered sugar. Use a spatula to fold in the powdered sugar. At first your mixture will turn thicker but as you continue to mix in the sugar, you should see your batter actually become thinner. Continue to stir until all powdered sugar is fully incorporated. Your batter should be very sticky and a little runny.
Scrape batter into your prepared pan. Use your spatula to spread it evenly across the pan as needed. Bake for about 20-25 minutes or until the toothpick inserted comes out with only a few crumbs clinging (mine took 20 minutes). Let brownies fully cool before cutting and serving.
Notes
Powdered sugar (especially if you are not using a fresh container) can often clump up. Make sure to break up the clumps with a whisk before measuring and adding it to your bowl. This will help ensure that you are measuring correctly and will make it easier for you to mix the sugar into the batter.
If you want to add a little chocolate flavor to these brownies, add 1 tbsp of unsweetened cocoa powder to the batter. Add it at the same time you add the powdered sugar.
If you don't have an 8 x 4 inch pan you can also use a 9 x 5 inch pan but your brownies will be a thinner and you will likely need less baking time.
I recommend lining the baking pan with parchment paper because the brownies are very fudgy and sticky at the bottom and it is hard to remove them from the pan directly.
I have only tested this with natural unsweetened peanut butter.
This recipe will not work with powdered sugar alternatives. Instead of melting into the brownies, the sugar becomes firm and hard.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.