I start by whisking the flour, sugar, and salt together in a large bowl. Then I cut in the cold, cubed butter using a pastry cutter until the mixture looks like coarse crumbs with pea-sized bits of butter. I slowly drizzle in the ice water, mixing just until the dough comes together without being sticky.
I divide the dough into two discs, wrap them in plastic, and pop them in the fridge for at least 30 minutes. This step matters more than people think, since chilled dough is what gives you those flaky, distinct layers once baked. Skipping it makes the dough harder to roll and less flaky overall.
While the dough chills, I beat the softened cream cheese with powdered sugar, vanilla, and lemon zest until it's completely smooth. I like to taste it at this point and adjust the sweetness slightly if needed. This filling should be thick enough to hold its shape once spread onto the dough.
Once chilled, I roll each disc out on a lightly floured surface to about ⅛-inch thickness. I trim the edges to make a neat rectangle, then cut it into even smaller rectangles, roughly 3x4 inches each. I make sure I have an even number so every bottom piece has a matching top piece.
I spread a spoonful of the cream cheese filling onto half of the rectangles, leaving a small border around the edges. Then I place the matching rectangle on top and press the edges together firmly with my fingers before crimping with a fork. This seals in the filling so it doesn't leak out during baking.
I place the assembled pop tarts on a parchment-lined baking sheet and chill them in the fridge for another 15-20 minutes. This second chill firms up the butter again, which helps the pastries hold their shape and puff up nicely in the oven.
I brush each pop tart with the beaten egg wash for a shiny, golden finish, then bake at 375°F for 18-22 minutes, until they're deep golden brown. I let them cool on a wire rack for at least 10 minutes before glazing, since a warm pastry will melt the glaze right off.
Once cooled, I whisk together powdered sugar, milk, and vanilla until smooth, then drizzle it generously over each pop tart. I let the glaze set for a few minutes before serving so it firms up slightly and doesn't run everywhere.