In a large bowl, whisk together flour, baking soda, cocoa powder, corn starch and salt. Set aside.

Combine chopped semi-sweet chocolate and butter and melt over a double boiler or in a microwave oven. Stir until smooth.

Mix in the sugars until mostly dissolved and then bring to room temperature.

Once cooled, mix the eggs and vanilla extract into the chocolate. 

Add the wet mixture into the dry ingredients and fold in until well combined.

Work in 1cup of chocolate chunks. cover the dough and refrigerate for 30 minutes

Preheat oven to 170 degrees C and line 2 baking trays with parchment paper.

Roll the dough into ping pong sized shapes and place on the baking paper. Press down gently with the palm of your hands.

Bake each tray in the oven for about 15 minutes until they spread out and start to crack.

Remove cookies from the oven and press in additional chunks of chocolate while still hot. Let the cookies cook for about 15 minutes before serving.