In a food processor, combine cassava flour and cold, cubed butter. Pulse until fully combined.
Transfer the flour/butter mixture to a large bowl. Then, add in the sugar, vanilla, salt and 2 tbsp of the cold water. Using your hands, mix everything together.
Add the remaining 2 tbsp of cold water and with your hands, mix again until a ball of dough forms. It should be slightly sticky, but not overly so and also not wet or grainy.
Place the ball of dough in the large bowl and chill in the fridge for 5 minutes.
Preheat the oven to 350° F.
While the dough chills just slightly, place two long sheets of parchment paper, side by side, on a cleaned counter top. Sprinkle cassava flour onto one of the pieces of parchment paper to roll the dough.
Remove the dough from the fridge and place it onto the piece of parchment paper with the flour. Sprinkle more cassava flour onto the dough ball, covering completely in a thin layer, then top with the clean piece of parchment paper.
Either using a rolling pin, a wine bottle, or even your hands, roll out the dough into a thin layer (about ⅛ in or so). Be sure not to roll it too thin as it could stick to the parchment paper/rip and not too thick as it could crack. If it does either, just try again!
Cut the dough into as many rectangles or squares as you can. You can opt to make either 8 smaller Pop Tarts or 4 larger Pop Tarts, depending on what is easier for you.
Spray a baking sheet with cooking oil (I use avocado oil spray) and place cut rectangles/squares on the baking sheet.
Take dough scraps and roll back into a ball, placing in the fridge to chill for 5 minutes. Once chilled, coat in flour and roll again. If the dough is sticking, it is either too thin or it needs more flour sprinkled on it. If it is cracking, use the warmth of your hands and fingers to mend together.
On half of the pieces of dough, put 1-2 tbsp (depending on the size/if you're making 8 or 4 Pop Tarts) of the hazelnut cacao spread in the center and spread out leaving a good amount of the edges clear.
Place the other pieces of dough on top of each piece with filling and press down the edge of the top piece onto the bottom piece. Use your finger to smooth and form the edges together as well.
Using a fork, press down on the edges to seal them together and poke holes all along the top over where the filling is.
Bake the Pop Tarts for 20-30 minutes, depending on the size, or until golden/golden brown. Note that white sugar or monkfruit will be golden and coconut sugar will be golden brown.
Once finished baking, move them to a cooling rack to cool to the touch.
Once they are completely cool, combine the frosting mixture and the hot fudge drizzle and decorate the top of each. Finish it off with some sprinkles if desired.
