Prepare the lemon curd by whisking the egg yolks, sugar, lemon juice, and lemon zest in a heatproof bowl until smooth.

Transfer the mixture to a saucepan and cook over low heat, whisking constantly, until it begins to thicken, about 5 minutes.

Add the butter gradually, stirring until fully melted and the curd coats the back of a spoon.

Remove from heat, strain for a smooth texture, then whisk in the heavy cream. Cover directly on the surface and chill until cold.

Make the crust by combining graham cracker crumbs, chopped nuts, melted butter, and sugar in a bowl.

Press the mixture firmly into the bottom of a greased 9-inch springform pan. Set aside.

In a mixing bowl, beat the cream cheese and sugar until smooth and creamy, about 2 minutes.

Add the heavy cream, lemon extract, and vanilla, mixing until blended.

Add the egg and mix just until incorporated, being careful not to overmix.

Pour the filling over the crust and smooth the top.

Wrap the outside of the pan with foil and place it in a larger baking dish.

Pour hot water into the outer dish until it reaches about 1 inch up the sides of the pan.

Bake at 350°F for 65 to 75 minutes, until the edges are set and the center gently jiggles.

Turn off the oven and let the cheesecake rest in the water bath for 20 minutes.

Remove and refrigerate for at least 2 hours, or until fully set.

Mix the sour cream, sugar, and vanilla until smooth, then spread evenly over the chilled cheesecake.

Spoon the lemon curd on top, leaving layers distinct or lightly swirling if desired.

Chill for another 1 to 2 hours before serving.

If using whipped cream, beat the heavy cream and sugar until soft peaks form.

Garnish with fresh berries and whipped cream just before slicing and serving.