Make the pecan graham cracker crust:

1. Preheat the oven to 350°F (177°C).
2. In a medium mixing bowl, whisk together chopped pecans, graham cracker crumbs, and sugar. Pour in the melted butter and mix until the texture resembles wet sand.
3. Transfer the crumb mixture into a 9-inch springform cheesecake pan and press it evenly into the bottom and halfway up the sides.
4. Bake for 11 minutes, then remove and set aside to cool while you make the cheesecake filling.

Make the turtle cheesecake:

1. Reduce the oven temperature to 325°F (163°C).
2. Prepare the springform pan. Wrap the bottom of the cooled springform pan with 2 sheets of aluminum foil to prevent water from seeping in during the water bath. Set aside.
3. Beat the cream cheese mixture. In a stand mixer fitted with the paddle attachment, beat softened cream cheese on low speed until smooth and creamy, about 2 minutes. Add brown sugar and caramel sauce, and continue mixing on low until fully combined and creamy, about 3 minutes.
4. Add eggs and incorporate slowly. While still beating on low, add the eggs and yolks one at a time, ensuring that each egg is fully incorporated before adding the next. Scrape down the sides of the bowl as needed to ensure everything is evenly mixed.
5. Add the remaining ingredients. Add sour cream, heavy cream, vanilla, cornstarch, and salt. Continue mixing on low speed until the filling is smooth and uniform. Avoid overmixing to prevent too much air from entering the batter.
6. Melt the chocolate. Place chopped chocolate in a microwave-safe bowl. Microwave in 30-second intervals, stirring in between, until completely melted and smooth.
7. Create the chocolate batter. Transfer one-third of the cheesecake batter into a smaller bowl. Add cocoa powder and melted chocolate, then mix until fully combined and smooth.
8. Layer the cheesecake batters. Pour one-third of the plain batter over the cooled crust in the springform pan. Add half of the chocolate batter on top. Repeat the layers, finishing with the plain batter on top. Optionally, swirl gently with a knife for a marbled look.
9. Bake in a water bath. Place the cheesecake in a large roasting pan and pour in enough boiling water to reach halfway up the sides of the springform pan. Carefully transfer to the oven and bake for about 1½ hours, until the edges are set but the center is still slightly jiggly. Begin checking at 1 hour 15 minutes, to ensure you do not overbake.
10. Cool gradually. Turn off the oven and crack the door open slightly. Let the cheesecake cool slowly in the oven for 1½ hours to prevent cracking. Then remove from the oven and water bath, and place on a cooling rack until it reaches room temperature.
11. Chill to set. Once cooled, cover the cheesecake with plastic wrap and refrigerate for at least 6 hours or overnight to allow it to fully set and develop flavor.

Make the topping:

1. In a small microwave-safe bowl, add the chocolate and heavy cream, then place into the microwave and heat in 30-second intervals, stirring in between, until melted, smooth, and glossy to form a ganache.
2. Remove the cold cheesecake from the fridge and top with caramel sauce, spreading evenly over the surface. Arrange pecan halves on top, then drizzle with more caramel and finish with a drizzle of the chocolate ganache.

3. Slice and serve chilled. For clean slices, dip a sharp knife into hot water and wipe dry before each cut.