Preheat oven to 350°F. Brush an 8” square baking pan with melted butter and line with parchment paper allowing the excess to hang over the sides. Brush the parchment paper with melted butter as well.
In a small mixing bowl, add flour, salt, 3 oz of chocolate chip (about ½ cup), and all of the peanut butter chips. Mix together evenly with a spatula. Set aside.
In a medium heat proof mixing bowl, add cocoa powder. Heat the water in a glass measuring cup in the microwave for 1 minute and immediately pour it over the cocoa powder. Mix with a whisk until all the cocoa powder dissolves. Add the remaining 1 oz of chocolate chip to the still-hot cocoa mixture and whisk until melted.
Melt butter in the microwave for about 30 seconds. Add the hot melted butter and vegetable oil to the chocolate mixture, and mix with a whisk until smooth.
Add the egg, egg yolk and vanilla extract to the mixture, and mix with a whisk again until smooth.
Add the sugar and whisk until completely incorporated.
Finally, add the flour mixture set aside earlier and fold with a spatula just until incorporated.
Pour the brownie batter into the prepared baking pan and bake for 40 minutes. A toothpick inserted in the center should come out clean.
Allow the pan to cool completely on a wire rack before unmolding the brownie and cutting into slices.
Melt the peanut butter in the microwave for about 30 seconds and drizzle over the sliced brownies if using.