Preheat the oven to 180°C. Line an 18cm x 28cm baking pan with baking paper.
To make the base, cream the butter and sugar with an electric beater until the mixture is light and fluffy. Add the vanilla essence and beat again. Sift in the flour, cocoa and baking powder and mix until evenly combined. Press into the base of the prepared pan and bake for 15 minutes.
Meanwhile, melt the butter, sugar and honey together in a saucepan over a low heat. Bring the mixture slowly to a boil and simmer for 2 minutes.
Sprinkle the nuts and chocolate over the hot base. Pour over the honey mixture.
Reduce the oven temperature to 150ºC and cook for another 10-15 minutes. Leave to cool in the pan. Cut into small pieces to serve.
For a chocolate pecan version, replace the mixed nuts with pecans.