1. Preheat the oven to 350°F 175°C. Line a 9×9 inch baking pan with parchment paper, leaving an overhang on two sides, and lightly grease the parchment and exposed pan edges.

2. Make the brownie batter by melting the unsalted butter in a saucepan or microwave safe bowl, then whisking in the granulated sugar and brown sugar until thick and glossy.

3. Whisk in the eggs one at a time until fully incorporated, then mix in the vanilla extract until the mixture is smooth and slightly thickened.

4. In a separate bowl whisk together the cocoa powder, flour, and salt until no lumps remain, then gently fold the dry ingredients into the wet mixture just until no dry streaks are visible.

5. Spread the brownie batter evenly into the prepared pan and bake for 15 to 18 minutes, until the top looks set around the edges but the center is still soft and slightly underbaked.

6. While the brownie base bakes, prepare the cheesecake filling by beating the cream cheese until completely smooth, then adding the granulated sugar and beating again until creamy and fluffy.

7. Mix in the sour cream, vanilla, and salt, scraping down the bowl as needed so there are no lumps left in the mixture.

8. Add the egg and egg yolk and beat on low speed just until combined, avoiding over mixing so you do not incorporate too much air into the cheesecake.

9. Fold in the chopped chocolate sandwich cookies, reserving a small handful to sprinkle over the top later.

10. Pour the cheesecake filling over the warm, partially baked brownie layer and smooth it into an even layer with an offset spatula.

11. Scatter the reserved cookie pieces evenly over the surface of the cheesecake layer for extra texture.

12. Return the pan to the oven and bake for 25 to 30 minutes, until the edges are set and the center still has a slight wobble when you gently jiggle the pan.

13. Remove the bars from the oven and let them cool completely at room temperature, then cover the pan and refrigerate for at least 3 hours or until thoroughly chilled.

14. Once the bars are chilled, make the ganache by warming the heavy cream in a small saucepan or microwave just until steaming but not boiling.

15. Pour the hot cream over the semi sweet chocolate chips in a heatproof bowl, let sit for 1 minute, then stir until the mixture is smooth, glossy, and lump free.

16. Spread the ganache over the chilled cheesecake layer, smoothing it into an even layer, and sprinkle the top with the crushed chocolate sandwich cookies.

17. Refrigerate for another 20 to 30 minutes to let the ganache set, then lift the slab out of the pan using the parchment overhang.

18. Use a sharp knife wiped clean between cuts to slice into 16 large bars or up to 20 smaller squares and serve slightly chilled or at cool room temperature.