Preheat oven to 350 degrees Lightly grease a 9×13 baking pan and line with parchment paper.
Sift together the flour, cocoa and baking soda. Set aside.
Melt the butter and mix together with the brown sugar, white sugar and vanilla in an electric mixer on low speed.
When combined add the eggs all at once. Note: at this stage you want the sugars to dissolve as completely as possible into the eggs butter and vanilla. I use my stand mixer on the lowest setting and leave it for 10 minutes or longer until no sugar crystals remain in the mixture. To test for this you can spoon a little of the mixture from the bottom of the bowl onto your thumb and then rub together your thumb and forefinger. You shouldn’t be able to feel any grains in the sugary syrup mixture.
Fold in the sifted dry ingredients and then the chocolate chips until the batter is smooth. Don’t overmix the batter; I prefer to do this sep by hand.
Pour about 1/2 or a little more of this batter into the prepared pan.
Begin to carefully spoon the cheesecake mixture over the brownie batter. Take your time and spoon it on gently, a tablespoon at a time. I use an offset spatula to smooth it at the end but this is not completely necessary.
Spoon the rest of the brownie batter on top of the cheesecake batter in the same careful manner and smooth the top with an offset spatula or the back of the spoon.
Bake for 45-50 minutes or until the top is well crackled and dry looking and the brownies are starting to shrink back from the sides of the pan. A toothpick inserted into the centre can still have a few wet crumbs left on it when withdrawn but not gooey in the centre. These are pretty thick cookie bars so they may take a little longer in your oven, depending upon the pans you use. I use all aluminum bakeware and they took the full 50 minutes.
Cool completely before cutting into cookie bars. Store in an airtight container. Freezes quite well too.
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