Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Spray or grease 12 muffin cups to prevent sticking. If you're feeling adventurous, you can even make mini muffin bites and adjust the baking time accordingly, about 8 minutes should do!
In a large microwave-safe bowl, combine granulated sugar, unsalted butter, and water. Microwave on high for about 1 minute until the butter is melted. Stir until the mixture is smooth and well combined.
Add in 3/4 cup of semi-sweet chocolate chips and stir until melted and creamy. Then, incorporate the large egg and vanilla extract, mixing until everything is nicely blended together.
Next, add the all-purpose flour and baking soda, stirring until just combined. It's important not to overmix; you want your brownies to be tender!
Let the batter cool at room temperature for a bit. This is crucial as it allows you to mix in the peanut butter chips and remaining semi-sweet chocolate chips without melting them.
Spoon the brownie batter into the prepared muffin cups, filling each one about three-quarters full. This will give them room to rise.
Bake for 13 to 15 minutes, or until the tops are set and a toothpick inserted into the center comes out slightly wet. Keep an eye on them, as they can go from perfect to overdone quickly!
Once baked, remove the pan and place it on a wire rack. You might notice that the centers of the brownies will fall a little upon cooling—this is normal! If they don't sink enough, lightly tap the centers with a teaspoon to create a small well.
In a small microwave-safe bowl, add creamy peanut butter and microwave on high for 45 seconds. Stir to make it smooth and creamy.
While the brownie cups are still hot, carefully spoon about a tablespoon of the warm peanut butter into the center of each brownie. This will create that gooey, irresistible center that everyone loves.
Finally, sprinkle some more semi-sweet chocolate chips and peanut butter chips on top for an extra touch of deliciousness. Allow them to cool completely in the pan before serving.