1. Prepare a 9 inch springform pan by lining the bottom with parchment and lightly greasing the sides. Wrap the outside of the pan in foil and preheat the oven to 350°F 175°C.

2. Make the brownie base by whisking together the melted butter and sugar until glossy, then whisk in the eggs and vanilla. Sift in the cocoa powder, salt, and flour and gently fold just until combined. Stir in the mini chocolate chips and spread the batter evenly in the prepared pan.

3. Bake the brownie layer for about 15 minutes, until the edges are set and the center is slightly puffed but still soft. Remove from the oven and reduce the temperature to 325°F 160°C. Let the brownie cool for about 10 minutes while you prepare the cheesecake filling.

4. For the cheesecake layer, beat the softened cream cheese until completely smooth. Add the sugar and beat until creamy and fluffy, scraping the bowl as needed. Mix in the sour cream and vanilla. Add the eggs one at a time on low speed just until incorporated, then beat in the cornstarch until combined.

5. Pour the cheesecake filling over the par baked brownie base and smooth the top. Place the springform pan on a baking sheet and bake at 325°F 160°C for 45 to 55 minutes, until the edges are set and the center still has a gentle wobble when the pan is nudged.

6. Turn off the oven, crack the door, and let the cheesecake rest inside for 30 minutes. Remove to a wire rack, run a thin knife around the edge, and cool to room temperature. Cover and chill for at least 4 hours or overnight until fully set.

7. When ready to finish the cake, make the ganache by heating the heavy cream until steaming, then pouring it over the chocolate chips in a heatproof bowl. Let sit for 1 minute, add the butter and pinch of salt, and whisk until smooth and glossy.

8. For the cookie nut crumble, stir together the chocolate cookie crumbs, chopped nuts, and melted butter until evenly coated and clumpy.

9. Release the chilled cheesecake from the springform pan and transfer to a serving plate. Pour the warm ganache over the top, nudging it toward the edges so it gently drips down the sides. Immediately sprinkle the cookie nut crumble over the ganache, concentrating more in the center. Chill for 15 to 20 minutes to help the ganache set.

10. Just before serving, gently warm the caramel sauce until pourable. Slice the cheesecake with a hot knife, wiping it clean between cuts, and drizzle each slice generously with caramel. Finish with a light sprinkle of flaky sea salt if desired and serve.