Step 1: Preheat oven to 350°F (175°C). Wrap the bottom and sides of a 9-inch springform pan tightly with heavy-duty aluminum foil to prevent water from seeping in during the water bath.
Step 2: Make the Crust: In a medium bowl, combine graham cracker crumbs, ¼ cup granulated sugar, and melted butter. Press the mixture firmly into the bottom of the prepared springform pan. Bake for 10 minutes. Remove from oven and let cool slightly.
Step 3: Reduce oven temperature to 325°F (160°C).
Step 4: Make the Filling: In a large bowl with an electric mixer, beat softened cream cheese on medium speed until smooth. Gradually add 1 cup granulated sugar, then beat in flour until just combined. Mix in vanilla extract and lemon zest (if using).
Step 5: Add eggs one at a time, beating on low speed after each addition until just incorporated. Do not overmix. Beat in the egg yolk, then gently fold in the sour cream until combined.
Step 6: Pour the cheesecake filling over the cooled crust in the springform pan.
Step 7: Prepare Water Bath: Place the foil-wrapped springform pan in a large roasting pan. Carefully pour hot water into the roasting pan until it reaches about halfway up the sides of the springform pan.
Step 8: Carefully transfer the roasting pan with the cheesecake to the preheated oven. Bake for 1 hour 15 minutes, or until the edges are set and the center jiggles slightly when gently shaken.
Step 9: Turn off the oven, crack open the door, and let the cheesecake cool in the water bath for 1 hour. This helps prevent cracking.
Step 10: Carefully remove the cheesecake from the water bath and roasting pan. Remove the foil. Let cool completely on a wire rack at room temperature.
Step 11: Once cool, cover loosely with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, until thoroughly chilled.
Step 12: Add Crème Brûlée Topping: Just before serving, sprinkle an even layer of ¼ cup granulated sugar over the top of the chilled cheesecake. Using a kitchen torch, caramelize the sugar until it forms a bubbly, amber crust. Keep the torch moving to prevent burning.
Step 13: Allow the caramelized sugar to cool and harden for a few minutes before slicing and serving. Enjoy!