Instructions:
- Prepare the 8x8 inch baking tin by lining it with parchment paper, ensuring there is an overhang on two sides to create a 'sling' for removal.
- In a medium saucepan over medium heat, combine the butter, peanut butter, granulated sugar, brown sugar, and milk. Stir constantly until everything is completely melted and smooth.
- Bring the mixture just to a simmer, then immediately remove the saucepan from the heat. Stir in the vanilla extract until fully incorporated.
- Measure the quick-cooking rolled oats into a very large mixing bowl. Pour the hot peanut butter mixture directly over the oats.
- Using a sturdy spatula, mix vigorously until all oats are evenly coated with the hot mixture. This requires thorough stirring to ensure consistency.
- Transfer the mixture immediately into the prepared tin. Use the spatula or the back of a lightly greased measuring cup to press the mixture down very firmly and evenly across the bottom of the tin. Firm pressing is crucial for setting.
- Place the tin in the refrigerator and chill for a minimum of 1 hour until the base is completely firm.
- If using the optional topping, melt the chocolate chips with the coconut oil gently over a double boiler or in short bursts in the microwave until smooth. Pour evenly over the chilled base.
- Return the tin to the refrigerator for another 30 minutes until the chocolate layer is fully set.
- Use the parchment paper sling to lift the entire block out. Slice carefully into 16 squares using a sharp knife, warming the knife blade slightly for cleaner cuts if necessary.