Line an 8×8-inch baking dish with parchment paper.
In a large mixing bowl, combine the peanut butter, honey (or maple syrup), and vanilla extract. Stir until smooth.
Add the oats and chia seeds to the peanut butter mixture and stir until evenly combined.
Fold in the chocolate chips (if using).
Add milk, one tablespoon at a time, until the mixture reaches a pressable consistency.
Press the mixture firmly into the prepared baking dish.
Chill the bars in the refrigerator for at least 2 hours, or until firm.
Remove the bars from the pan and slice into desired sizes.
