Preheat oven to 350°F. Line an 8 inch square baking pan with parchment paper and allow it to slightly overhang at the opposite ends. Coat the paper with cooking spray.
In a mixer fitted with a paddle attachment, beat yogurt,oil and hazelnut butter until creamy. Add in eggs and brown sugar and beat them until combined.
Combine oat flour, baking powder and salt in a separate bowl. Add the flour mixture into the wet ingredients and mix them until just combined. Add the chocolate chips and gently stir them in.
Pour and spread the batter evenly into the prepared pan and bake for 25 minutes. Check to see if the brownies are done using the toothpick method: a toothpick inserted into the center of the pan should come out clean.
Remove to a rack to cool for around 30-45 minutes and cut them into squares.
Let cool completely before storing them in the refrigerator. Consume within 3-4 days.