Preheat oven to 375 °F (191 °C). Line a 9-by-13 inch pan with parchment paper with a 2-inch overhang on either side.
In a large bowl, combine graham cracker crumbs, sugar, and butter. Mix together until crumbs are moistened and when you grab a clump, it forms a shape - kind of like wet sand. Press the graham cracker crumbs into the bottom of the prepared pan. Bake for 10 minutes. Remove and set aside. Lower oven temperature to 350 °F (177 °C)
1 1/2 cup graham cracker crumbs, 1/4 cup granulated sugar, 6 tablespoon unsalted butter
In the bowl of a stand mixer with the paddle attachment, beat together cream cheese, egg, sugar, milk, vanilla extract, and flour. Gently pour cheesecake batter on top of the graham cracker crust. Bake for 25-30 minutes until just set. Remove from oven but keep oven temperature. Let cheesecake cool for 20 minutes.
In the meantime, combine hazelnuts, pecans, corn syrup, butter, dark brown sugar, egg, and salt in a bowl. Stir together until everything is combined well.
1 cup chopped hazelnuts, 1 cup chopped pecans, 1/2 cup light corn syrup, 4 tablespoon unsalted butter, 3/4 cup dark brown sugar, 1 egg, Pinch of salt
Carefully spread the nut mixture on top of the slightly cooled cheesecake layer. The middle will likely fall in and cheesecake may ooze out around the nut filling, but it's ok, just gently keep spreading it across the entire pan!
Bake for another 25 minutes.
Remove from oven and let cool completely. Place in the refrigerator overnight to set and slice into squares before serving.