Preheat the oven to 350F and grease and 8x8 baking pan with butter (or oil).
In a large mixing bowl, cream together the butter and sugar using a hand- or stand-mixer on high until smooth and fluffy. Add in the egg, nut butter and vanilla - one at a time - and beat again until incorporated. Set aside.
Combine the dry ingredients in a small bowl and stir. Slowly scoop spoonfuls of the dry ingredient mixture into the large bowl of butter-sugar-egg mixture. Keep the mixing speed on low to prevent flour from flying everywhere. Mix until just combined and gently fold in the chocolate bar chunks and nuts.
Transfer the batter into the greased baking pan and lightly press it to evenly spread throughout the pan. Sprinkle with chocolate chips and bake for 20-22 minutes, or until golden brown and a toothpick comes out clean.
Cool before slicing. Store in an airtight container at room temperature or in the fridge.