Line a shallow baking dish with parchment paper (or wax paper). I used a 7×11 glass dish.
Place pitted dates in a bowl. Cover with boiling or very hot water (I used my tea kettle). Allow them to soak for at least 10 minutes.
Place soaked dates plus 5 tbsp of the soaking water in a blender (I used my nutribullet). Blend until combined. The mixture will be very thick.
Place date mixture, peanut butter, brown rice syrup, and vanilla in a small saucepan over medium heat. Stir until well-combined and heated through.
In a large bowl, combine oats, chia seeds, and flax. Stir in wet mixture until combined.
Spoon mixture into baking dish, pressing very firmly so mixture is evenly spread throughout pan. It should be very compact. Place another piece of parchment paper on top and press down firmly. Remove top piece of parchment and freeze bars for at least 2 hours.