Preheat your oven to 180°C / 350°F.
Line a 9x9 baking dish with parchment paper.
With an stand mixer fitted with a paddle attachment, or a hand-held electric mixer, mix the brown sugar and peanut butter on medium speed until until well combined, about 3 minutes. Add the vanilla.
½ cup light brown sugar, 1 cup natural peanut butter, 2 teaspoons vanilla extract
Add in the flour, oats, salt, and baking soda. The dough will be very thick and clumpy, but it should hold together if you squeeze a handful of it.
1 cup white whole wheat flour, 1 cup rolled oats, 1 teaspoon baking soda, ¼ teaspoon salt
Slowly add the milk in a steady stream, mixing until a clumpy dough forms.
½ cup milk
Fold in chocolate chunks, using your hands to knead everything together.
¾ cup dark chocolate chips or chunks
Press the dough into the prepared baking dish - wet hands are helpful here
Bake for about 15-20 minutes until the bars are lightly golden on top and set.
Allow to cool completely and cut into squares.
Once cooled, drizzle with melted dark chocolate if desired.
extra dark chocolate for drizzling