Soak lentils for up to 12 hours if possible, but even 1 will do! Whether or not you soak, rinse lentils very well and combine with 4 cups / 1 liter water in a small saucepan. Bring to a boil, and cook for 15-20 minutes semi-covered, stirring occasionally until completely soft.
Drain in a fine mesh sieve, shaking vigorously to remove most of the cooking water / starch. Drain until you have 1 ½ cups / 450g cooked lentils remaining, discard the cooking liquid.
Preheat oven to 350°F / 175°C.
Place the lentils in a food processor and blend on high for 60 seconds - the lentils should be whipped and airy. Add the nut / seed butter, vanilla, and blend again until fully combined. Add the dry ingredients: sugar, cacao, flax, baking soda, and salt. Blend thoroughly. Remove the blade from the machine, and fold in most of the chopped chocolate by hand, reserving a few large chunks for the top, if desired.
Transfer batter to a lightly greased, 8” x 8” brownie pan. Smooth out the top. Add reserved chocolate chunks on top of the batter and sprinkle with flaky salt.
Bake for 30-35 minutes until a toothpick comes out clean. Remember, fudgey centres are encouraged and be mindful that you may be testing in a melted chocolate chunk zone ;)