Preheat your oven to 175°C (350°F).
Line an 8x8-inch baking tin with baking paper.
Add the chopped dark chocolate and coconut oil to a heatproof bowl. Place the bowl over a saucepan of gently simmering water and stir until the chocolate and coconut oil have completely melted and combined.
Mash the ripe avocados in a large mixing bowl until smooth. Add the melted chocolate mixture and stir well to combine.
Whisk in the coconut sugar, flax eggs and vanilla extract until fully mixed.
Add the almond meal, cocoa powder, baking powder and sea salt. Mix gently until just combined, being careful not to over-mix.
Fold through the chopped walnuts.
Spread the brownie batter evenly into the prepared baking tin.
Place in the centre of the oven and bake for 25–30 minutes, or until the middle is set.
Allow the brownies to cool completely on a wire rack before slicing into squares.