Preheat your oven to 350.
First, let's start off by making the dough. Place all the dry ingredients in a food processor and pulse until well mixed.
Add in the cubed butter and pulse until a coarse crumb comes together and the butter has been broken down. Ensure there are no large clumps of butter remaining.
Lastly, add in the egg yolk and process on low until the dough comes together and the egg has been processed into the dough well.
Remove the dough from the processor and knead it together slightly to form a disk. Line a sheet pan with parchment paper, then press the dough into the sheet pan, ensuring it is spread evenly and completely.
Now, using a sharp knife, cut a line horizontally across the middle of the dough. This will become your "folding" line, where you will fold the dough over itself to create a large pocket with the goodness in the middle. Place the sheet pan in the freezer.
Now, let's prepare the chocolate ganache/fudge filling. Simply place your chopped chocolate or chocolate chips in a glass heatproof bowl, along with the coconut milk. Heat in the microwave in 20 second intervals, stirring in between until the chocolate has melted completely and the mixture is smooth.
Set the ganache aside to cool and firm up. Place the ganache into a piping bag with a large circle piping tip.
For the meringue: Create a double boiler. Add the egg whites, sugar and cream of tartar to the heatproof bowl, then using the double boiler, place on the stovetop over medium heat.
Whisk the whites and sugar constantly until the egg white mixture has thinned out and the sugar has dissolved completely. This should take about 4-5 minutes. To check if it is done, scoop a small amount of the mixture onto a spoon. Once it has cooled just slightly, dip your fingers into the mixture and rub it between your fingers. If you cannot feel any sugar granules, it is done cooking. If it still has even a slightly grainy texture, continue to heat the mixture. The mixture will also be lighter in color and more voluminous with a foamy look on top.
Once the sugar is completely dissolved, pour this mixture into a bowl on a stand mixer fitted with the whisk attachment. Beat the egg whites on medium low speed for 2-3 minutes and gradually increase the speed until the mixture reaches soft peaks and the bowl is no longer warm to the touch. Then increase the speed to high and beat until stiff peaks form. The meringue should be shiny, bright white, and hold its form well.
Allow to cool if need be, then place the cooled meringue in a piping bag with a large circle piping tip.
Lastly, let's whip together the chocolate glaze frosting. Add the melted butter, powdered sugar, cocoa powder and coconut milk to a bowl and whisk until smooth and shiny.
To assemble: Pipe on the meringue to one side of the pastry dough in rows, leaving an inch between each row and an inch border around the crust. You will want to leave a 1 in border so that you will be able to crimp together the crust, making those quintessential pop tar crimped edges. Make sure to only pipe the filling onto the 1 half of the crust, as you will fold the other half on top.
In between the rows of meringue, pipe on rows of the ganache/fudge filling, continuing to leave a 1 inch border all the way around.
Next, carefully fold the plain crust side, directly on top of the filled crust. Adjust as needed to ensure the crust is lined up evenly. This step works best when the dough is still cold.
Now, you are going to go around the edges and crimp the dough together. It is best to let the crust warm and thaw a bit before this step so that it doesn't crack while crimping. Using a fork, go around the entire border crimping the crusts together. Bake for 25 minutes until lightly golden brown.
Allow the giant pop tart to cool completely, then generously pour the frosting in the center and spread to cover the pop tart, leaving the crimped border unfrosted.