Place the heavy cream, sugar and salt in a small saucepan on medium heat for two minutes, stirring. Add the chocolate and continue to stir for another three minutes. Remove from heat and stir in the butter followed by the vanilla. Stir until smooth. 

Pour mixture into a bowl and refrigerate until it thickens but is still spreadable. 

Chop butter into cubes and place in the fridge while you assemble the rest of the ingredients.

Fill a small bowl with some cold water and an ice cube. Stir the ice cube around a bit and set bowl aside.

In a food processor, pulse together the flour, cocoa, sugar, and salt. Add the cubed butter and pulse until the dough forms pea sized balls. Add ice water a tablespoon at a time while continuing to pulse mixture until it is just combined.Wrap the dough tightly with plastic wrap and refrigerate it for at least 30 minutes. Once the dough has chilled for at least a half hour, leave it out at room temperature for 20 minutes before rolling it out.

Turn out dough onto a lightly floured surface.  Turn dough as you roll it out to make sure it is not sticking to your surface. Roll out to a 1/8-inch thickness. Use a sharp knife or pizza cutter to cut dough into 4 x 3 inch rectangles or use a cookie or biscuit cutter (I used a 3 inch round cutter) to make mini pop tarts. Place one of the rectangles (or circles) on a baking sheet lined with parchment paper. Spoon about a tablespoon of chocolate filling on one half of the rectangles (or half a tablespoon if making mini pop tarts). . Spread the filling using the back of a spoon or a knife. Leave about a 1/4 inch border and brush the edges with the egg wash.

Lay another half of the dough on top and gently seal the edges with your fingertips then crimp the edges with the back of a fork. Poke the tops of the pop tarts with a fork or a tooth pick to create tiny air vents. 

Continue until all the dough has been used. Dough can be rolled out a second time (or third, if necessary). Cover the baking sheets and place in the fridge for 30 minutes.

Preheat the oven to 350 degrees F. Bake the pop tarts for 20-25 minutes  (17-20 minutes for mini pop tarts and 20-25 minutes for regular pop tarts). Allow pop tarts to cool completely on a wire rack before frosting. 

Stir until smooth the melted white chocolate, powdered sugar, vanilla (if using) and cream or milk. Start with 1 tablespoon of cream or milk and continue adding until icing is the consistency you like. Drizzle or spoon over cooled pop tarts. Top with some turbinado sugar while icing is still wet, if desired!