Preheat oven to 200C (400F).
Roughly chop the dark chocolate if you do not use chocolate chips.
In a bowl, mix flour, soda, and salt.
In a separate bowl mix butter and both sugars until creamy, about 3-4 minutes, add in egg and vanilla, whisk to incorporate.
Add in dry ingredients and mix quickly to form a dough. Add in the chopped chocolate (reserving some for garnish) mix quickly until the chocolate has spread evenly into the batter. Set aside while you prepare a filling
Mix cream cheese, sour cream or creme fraiche, eggs, sugar, and vanilla until incorporated and smooth.
Cover the bottom of the 33x23cm (13x9) baking pan with parchment paper. Spread 2/3 of cookie dough evenly on the bottom of the pan. Pour cream cheese filling and crumble the rest of the dough over cheese filling. Sprinkle reserved chocolate chunks/chips over the dough.
Bake around 30-40 minutes, until it has a nice golden brown color.
Let it cool completely in the refrigerator before cutting it into pieces and serve.