Preheat your oven to 350°F (175°C) and grease a 9x9-inch baking pan.
In a medium bowl, melt chocolate chips and butter together.
Stir until smooth, then remove from heat.
In another bowl, whisk together sugar and eggs until creamy.
Add the melted chocolate mixture to the egg mixture and combine.
Stir in flour, salt, and vanilla extract.
Fold in peanut butter and mix until combined.
Pour the batter into the prepared baking pan and spread evenly.
Bake for 25-30 minutes or until a toothpick comes out with a few moist crumbs.
Let cool before cutting into squares and serving.