Preheat oven to 160°. Grease and line a 20x30cm slice tin.
In a large bowl, stir together the dates, baking soda and boiling water. Add the butter and mix until melted. Leave for 20 minutes to allow the dates to soften.
With a fork, mash the softened dates to break them up to a smoother consistency. Add the eggs and vanilla and whisk until combined.
Whisk in the sugar, then add the sifted cocoa, flour, and baking power and stir until smooth. Add the chocolate chunks (and nuts, if you wish)
Pour evenly into slice tin, smoothing the batter into the edges. Bake about 45 minutes, or until skewer inserted in the middle comes out almost clean and springs back to touch.
Leave in the tin to cool before cutting into squares.