Preheat your oven to 170C/338C. Line a deep 18CM by 27CM baking tray with baking paper and grease with oil.
In a small mixing bowl mix ground flaxseed or chia seeds with boiling water together to form a paste. Set aside.
In a large heatproof bowl melt butter and chocolate together in the microwave in short bursts or on the stove in a saucepan. If you want a chewier denser brownie, add 25g of extra butter. Set aside for a few minutes to cool.
In another mixing bowl, sieve together flour, cocoa, sugar, baking powder and salt. Give it a good whisk to combine. Add half the amount of chocolate chips.
Add flaxseed paste to butter chocolate mix. Stir with a whisk until combined.
Pour into flour mixture, and fold through with a spatula or wooden spoon.
Pour mixture into your baking tray and spread out evenly in the tray using your spatula.
Add remaining chocolate chips over the top and gently push into the batter.
Bake on the top rack of your oven for 35-40 minutes. You want the top to be slightly firm and the centre will still be slightly gooey. Once out of the oven allow to sit for 30 minutes so it can become firm. Slice and serve.