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1
Preheat the oven to 180C/350F/gas mark 4, then lightly grease the brownie tin (18cm square) and line the base with baking parchment.
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2
Whisk together the ground flaxseeds, baking powder and rice milk until blended, then set aside for a few minutes.
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3
The flax will swell up and absorb the rice milk so that you are left with a thick paste rather than a liquid.
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4
Place a heatproof bowl over a saucepan containing 2.5cm/1in of simmering water, break up the chocolate and place it in the bowl with the margarine.
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5
Melt gently, stirring occasionally, until smooth and combined, then remove from the heat and set aside to cool down a little.
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6
Meanwhile, roughly chop the nuts.
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7
Once the chocolate is cool enough to touch, stir in the sugar, flaxseed mixture and vanilla extract until fully amalgamated.
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8
Transfer to a large bowl and then sift in the flour, folding it in with the chopped nuts and a very small pinch of salt (which will do wonders for the intensity of the chocolate).
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9
Pour the mixture into the prepared tin, spreading it out evenly and levelling the top with the back of a spoon, then bake in the oven for 25-30 minutes or until the top has crisped up slightly but the cake is still gooey in the middle.
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10
(You can use a skewer or cocktail stick to test this.)
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11
Leave to cool in the tin completely before cutting into squares and serving.
