Preheat your oven to 350°F (175°C) and wrap a 9-inch springform pan tightly with aluminum foil.

Blend the walnuts in a food processor, then combine with graham cracker crumbs, brown sugar, salt, and melted butter, press into the bottom of the pan, and bake for about 10 minutes until golden.

Lower the oven temperature to 325°F (160°C). Beat together the cream cheese and sour cream, then add sugar and vanilla until combined. Incorporate eggs one at a time.

Stir in maple syrup and extract, then add heavy cream and fold in flour until smooth.

Prepare a water bath in a roasting pan, bake cheesecake for 1 hour and 20 minutes. The edges should be firm and the center jiggly.

Turn off the oven, leave the cheesecake inside with door ajar for 30 minutes, then cool completely at room temperature.

Refrigerate the cheesecake for at least 6 hours or overnight for best results.

Melt butter in a skillet, add brown sugar until bubbly, then stir in cream, salt, walnuts, vanilla, maple syrup, and bourbon if using.

Cool the topping slightly, pour over cheesecake, slice, and serve.