In a large mixing bowl, whisk together flour, sugar, and salt. Add cold cubed butter and work into flour until mixture resembles coarse crumbs with pea-sized pieces.

Gradually add ice water one tablespoon at a time, mixing with a fork until dough just comes together. Shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes.

Prepare cinnamon sugar filling by combining granulated sugar, brown sugar, cinnamon, and cornstarch in a small bowl. Mix thoroughly and set aside.

Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.

Divide chilled dough in half. Roll out each half to 1/8-inch thickness and cut into 3x4 inch rectangles.

Place half the rectangles on prepared baking sheets. Spoon 1.5 tablespoons cinnamon sugar filling onto center of each, leaving 1/2-inch border.

Beat egg with 1 tablespoon water. Brush borders with egg wash, then top with remaining pastry rectangles.

Press edges firmly with a fork to seal. Poke a few small holes in tops with a toothpick. Brush tops with egg wash and sprinkle with coarse sugar.

Bake for 25-30 minutes until golden brown and crispy. Cool on baking sheets for 5 minutes, then transfer to wire rack to cool completely.

For chocolate drizzle, heat heavy cream until steaming. Pour over chocolate chips, let sit 1 minute, then stir until smooth. Cool 5-10 minutes until slightly thickened.

Drizzle chocolate over cooled pop tarts in zigzag pattern. Let set for 15 minutes before serving.