Cut in cold butter until pea-sized crumbs form.
Add ice water 1 tablespoon at a time just until dough comes together.
Divide dough into 2 discs, wrap, and chill for 30 minutes.
Beat cream cheese, ube halaya, powdered sugar, vanilla, and salt until smooth.
Preheat oven to 375°F (190°C) and line a baking sheet with parchment.
Roll out dough to about 1/8-inch thick on a floured surface.
Cut dough into 3x5-inch rectangles (re-roll scraps as needed).
Place half the rectangles on the baking sheet.
Spoon 1 1/2–2 tablespoons filling onto each, leaving a border.
Brush edges with egg wash.
Top with remaining rectangles and press to seal.
Crimp edges with a fork.
Brush tops with egg wash.
Poke 1–2 small vents in the top of each pop-tart.
Bake 22–25 minutes until lightly golden and set.
Cool on the pan 5 minutes, then transfer to a rack to cool completely.
Whisk powdered sugar, milk/cream, and ube halaya until smooth for the glaze.
Spread glaze over cooled pop-tarts and let set before serving.