For topping, in a large saucepan, combine 4 egg yolks, evaporated milk and vanilla and whisk until well combined. Add sugar and 3/4 cup butter and cook on medium heat for 12-15 minutes or until thickened (almost pudding-like) and golden brown, stirring constantly. Remove from heat and add coconut and pecans. Mix well. Allow to cool completely. Set aside.
For crust, heat oven to 325 degrees. In a small bowl, combine Oreos and 4 Tbsp. melted butter and mix well. Press mixture into the bottom and up the sides of a 9-inch springform pan. Bake crust for 10 minutes then remove and set aside. Cover the outside of the pan with aluminum foil and set aside.
For cheesecake filling, reduce oven to 300 degrees. In a large bowl, blend the cream cheese, sugar, chocolate, flour and cocoa with an electric mixer until combined. (Use low speed to keep less air from getting into the batter which can cause cracks.) Scrape down the sides of the bowl. Add 4 eggs one at a time, beating slowly and scraping the sides of the bowl after each addition. Add the sour cream and vanilla extract. Beat on low speed until well combined. Put a little bit of cheesecake filling in bottom of crust. Add about 1 1/2 cups of coconut pecan topping and spread evenly. Pour remaining cheesecake filling into the springform pan and spread evenly. The pan will be very full. Place springform pan inside another pan. Fill outside pan with enough warm water to go about halfway up the sides of the springform pan. The water should not go above the top edge of the aluminum foil on the springform pan. Bake for 1 hour and 30 minutes. Turn off heat and leave cheesecake in the oven for another 20 minutes or until set. This cooling process helps the cheesecake cool slowly to prevent cracks. Remove cheesecake from oven and chill. Once the cheesecake is completely cool, remove springform pan sides from cheesecake and place cheesecake on a serving plate. Top cheesecake with remaining coconut pecan topping and drizzle with chocolate sauce.