Pre-heat oven to 350F (180C), butter an 8x8 inch (20 x 20cm) square pan, then line the pan with parchment paper and lightly butter the parchment paper again.
In a small bowl over a pot with boiling water (make sure the water does not touch the bottom of the bowl), melt the chocolate and butter (stir gently until melted and smooth) set aside to cool.
In a large bowl add the sugar, eggs, milk, vanilla and the salt and beat on medium speed for 5-7 minutes or until thickened and pale. Then add the melted cooled chocolate and beat 30 seconds, then sift the flour, cocoa and baking powder directly into the bowl and zest, fold gently to combine.
Pour into prepared pan and bake 30-35 minutes or until it begins to pull away from the edges. Let cool and dust with powdered/icing sugar if desired. Enjoy!