Step 1: Place the bread flour, wholemeal flour, water, active levain, cocoa powder, sugar and ground cardamom into the bowl of a mixer fitted with the paddle attachment. Mix well until it comes together as a dough. Cover the bowl and rest for 30 minutes.
Step 2: Add salt to the dough and mix for 5 minutes on medium speed. Add the walnuts and chocolate and mix until thoroughly combined.