Grease or line an 8" square baking with parchment or wax paper.
Place a medium sauce pan over medium heat. Add peanut butter and stir until melted. Mix in the ground flax, maple and vanilla until thoroughly incorporated.
Pour mixture into baking pan, even out with spatula if necessary. Refrigerate, letting the peanut butter set and thicken. About 20 minutes.
In a microwave safe bowl, add the chocolate and 4 tbsp peanut butter. Microwave, stirring and checking every 15 seconds, until complete melted.
Pour over the top of peanut butter mixture. Place back in refrigerator.
When the chocolate starts to firm up a bit, sprinkle over the flaked sea salt. Continue to refrigerate until bars are completely set.
Pull paper up and transfer the bars to a cutting board. Cut into squares.