In a large bowl, combine butter and brown sugar. Add the peanut butter and stir to combine. Mix in the eggs and vanilla. Stir well.
In a separate bowl, combine all dry ingredients. Stir into the batter.
Pour in the Reeses pieces and chocolate chips. Gently mix.
Evenly spread batter into a greased 9×13 pan.
Bake at 350 for about 30 min or until an inserted toothpick comes out clean.
Allow to cool.