a piping bag fitted with a small open tip (Ateco number 3)
For the crust: Preheat the oven to 350 degrees F. Spray an 8-inch springform with nonstick cooking spray. Wrap the springform in 2 layers of aluminum foil. It's best to use the widest type of foil you can find, one that can be uniformly wrapped up the sides of the springform without leaving a gap near the bottom of the pan. Place the springform in a cake pan or hotel pan wider than the springform. Set aside.
Pulse the cookies, sugar and salt in a food processor until the mixture resembles a fine powder. Slowly add the melted butter and pulse until the mixture starts to clump.
Transfer the mixture to the prepared springform and gently press the cookie mixture into the bottom of the pan. Use a small measuring cup to evenly tamp down the edges. Bake for 10 minutes and allow to cool slightly while you prepare the filling.
For the filling: Put the sweet potatoes in a saucepan and cover with water. Bring to a simmer and cook until soft, about 15 minutes. Strain, transfer to a food processor and pulse until the sweet potato becomes a smooth puree. Measure out 1 cup sweet potato puree and set aside. (Reserve any remaining puree for another use.)
In the bowl of a stand mixer fitted with the paddle attachment, add the softened cream cheese. Mix on low until smooth. Add the sugar and mix until just combined. Don't overwhip.
Whisk together the eggs in a small bowl and add to the stand mixer bowl. Mix until just combined. Add the cream, extracts, speculoos spice and salt. Sift the flour over the mixture and fold until the flour is incorporated and the mixture is smooth. Transfer 1/4 cup of the mixture to a piping bag fitted with a small open tip (Ateco number 3). Set aside.
To the remaining batter, add the sweet potato puree and orange zest and fold into the batter with a rubber spatula until completely incorporated. Transfer the batter to the springform on top of the crust. Pipe the reserved batter into leaf shapes on top and pull a toothpick through the leaves.
Transfer the pans to the preheated oven onto the middle oven rack. Carefully pour hot water into the larger cake pan or hotel pan so it comes just halfway up the springform. Bake until the batter just shimmies in the middle of the cheesecake, 1 to 1 1/2 hours. Turn off the oven and open the oven door. Allow the cheesecake to cool in the cooling oven, about 2 hours.