Preheat your oven to 140 C or 300 F . Line a baking tray with parchment or grease with butter.
Whisk together the flour, cocoa , baking soda and salt.
With a mixer or a wooden spoon , cream the butter and sugar until light and fluffy. This will take 3-5 minutes with a mixer and longer by hand.
Mix in the eggs and vanilla until smooth.
Add in the dry ingredients and fold .
Mix in the chocolate chips
Shape into two logs (each about 25 x 5 cms) . Place on baking sheet and bake at 140 C / 300 F for 30 minutes. It ll be firm but spring back on touching
Cool for 10 minutes.
Slice obliquely (about an inch thick) using a serrated knife. you can use a sawing motion. I just cut straight down.
Place the slices on a unlined and ungreased baking sheet. Bake at 140 c/ 300 F for 15 minutes .
Turn the slices over and bake for another 15 minutes. Cool completely