Preheat the baking oven. Line a large baking tray with parchment paper or the aga liner.
In a bowl, sieve together the flour, cocoa powder, baking soda and salt.
In a separate bowl, beat together butter and sugar until light and fluffy. Add eggs and vanilla extract and beat until combined well. Stir in flour mixture to form a stiff dough then add in chocolate chips and stir until combined.
Divide the dough in half.
Lightly flour a work surface and dust your hands too, then shape the dough into two slightly flattened logs on to the prepared baking trays, each approximately 12 inches long and 2 inches wide. Dust with icing sugar.
Place in the baking oven and bake for 30 - 35 minutes, or until slightly firm to the touch. Remove from the oven and set aside to cool for 5 minutes.
Transfer to cutting board and cut diagonally into about 1 to 2 cm slices.
Arrange the biscotti, cut sides down, on baking sheet and return in the simmering oven for about 20-30 minutes, or until crisp. Cool on a rack.
Serve the biscotti with your favourite dessert or coffee.