For the chocolate fudge filling:

1. In a saucepan, mix together the cocoa powder, sugar, milk, salt, and half of the chopped chocolate. Heat over medium-low, stirring, until the chocolate has melted and you have a smooth mixture.
2. Remove from heat and add the butter and the remaining chopped chocolate. Stir until both have melted and the mixture is glossy. Set aside — the filling will firm up slightly as it cools.

For the chocolate pie dough:

1. Whisk together the gluten-free flour, cocoa powder, sugar, and salt in a large bowl.
2. Add the cold cubed butter and rub into the flour mixture with your fingertips (or use a pastry cutter) until the mixture resembles coarse crumbs with some pea-size pieces remaining.
3. Add ice water one tablespoon at a time, mixing gently with a fork after each addition, until the dough just comes together and holds when pinched. Do not overwork.
4. Shape into a flat disk, wrap in cling film, and refrigerate for at least 30 minutes (or up to 2 days).

Assembling the pop tarts:

1. Preheat oven to 355°F (180°C) and line a baking sheet with parchment paper.
2. Roll out the chilled dough to about 3 mm thick and cut out 3 x 4-inch rectangles. You should get 16 rectangles (makes 8 pop tarts).
3. Brush the edges of half the rectangles with egg wash. Spoon a heaped tablespoon of the chocolate fudge filling into the centre of each. Spread slightly, leaving at least 3/4 inch around the edge.
4. Prick the centres of the remaining 8 rectangles with a toothpick. Place these over the filling, seal the edges with your fingers, and crimp with a fork.
5. Bake for about 20 minutes, until slightly puffed and flaky. Allow to cool slightly.

For the chocolate frosting:

1. Mix together the powdered sugar and cocoa powder. Slowly add water 1/2 tablespoon at a time, whisking after each addition, until the frosting gently drips down the sides but still forms a generous layer on top of each pop tart.

Decorating:

1. Spoon the frosting onto the slightly cooled pop tarts. If you want them extra pretty, direct the flow of frosting with a spoon. Scatter with chocolate sprinkles.

Storage:

1. Keep in a closed container in a cool, dry place for 3–4 days.