Melt chocolate chips in a large bowl. This is most easily done in the microwave by heating in 30-second intervals, mixing between each interval until the chocolate chips are almost all melted, stirring to melt the remaining chips.
Add sugar, oil, and vanilla extract. Mix to combine.
Add eggs, one at a time, mixing well between each addition.
Sift together dry ingredients.
Add to the wet ingredients and mix until well combined and the batter is smooth.
Pour into a greased or lined 9x13 pan. Drop tablespoons of peanut butter throughout the brownie batter. Swirl through with a butter knife.
Bake for 30-35 minutes. If you like more fudgy brownies, remove them from the oven when a toothpick comes out almost clean. If you like chewy brownies, remove them from the oven when the toothpick comes out clean.