In a saucepan, mix together the cocoa powder, sugar, milk, salt and half of the chopped chocolate. Heat until the chocolate has melted and you get a smooth mixture.
Remove from heat and add in the butter and the rest of the chopped chocolate. Mix everything together until the butter and chocolate have melted.
Set aside. The chocolate fudge filling will firm up slightly while cooling.
Sift together the gluten free flour, coconut flour, cocoa powder, xanthan gum, salt and sugar.
Add the butter and toss it in the flour until all butter pieces are covered with it. Using your hands, pinch together the butter pieces and flour until you get a mix of fine pea-sized pieces and larger thumb-sized pieces.
Add the ice cold water-vinegar mixture, 2 tbsp at a time, and mix the pie dough with a fork until it comes together. You will need 14 – 16 tbsp of water.
The dough at the end should be slightly more damp/wet than typical pie crust because gluten free flour tends to absorb more moisture during baking, so we want to counteract that.
Shape the chocolate pie dough roughly into a rectangle (if needed, knead it slightly – but don't overwork it!), wrap it into cling film and refrigerate for at least 1 hour.
Pre-heat the oven to 355 ºF (180 ºC) and line a baking sheet with baking/greaseproof paper.
Roll out the chilled pie dough about 3 mm thick, and cut out 3 x 4 inch rectangles. You should get 16 rectangles (makes 8 pop tarts).
Egg wash the edges of half of the rectangles and spoon a heaped tablespoon of the chocolate fudge filling into the centre of each one. Spread the filling slightly, but leave at least 3/4 inch room around the edge.
Prick the centres of the other 8 rectangles with a toothpick. Place these rectangles over the filling, seal the edges with your fingers and "crimp" the edges with a fork.
Bake the pop tarts in the pre-heated oven at 355 ºF (180 ºC) for 20 - 22 minutes.
Allow to cool slightly.
While the chocolate pop tarts are baking, make the chocolate frosting by mixing together the powdered sugar and cocoa powder. Add the water, 1/2 tablespoon at a time while stirring/whisking, until you get the right consistency. (The right consistency in this case means that the frosting will gently drip down the sides, but still form a generous layer on top of each pop tart.)
Spread the frosting onto the slightly cooled pop tarts and decorate with chocolate sprinkles.
The gluten free triple chocolate pop tarts keep well in a closed container in a cool dry place for 3 - 4 days.