In a large bowl, whisk whole-wheat flour, all-purpose flour, sugar, and salt. Cut in the cold butter with a pastry cutter or your fingertips until pea-sized bits form. In a small bowl, whisk the egg, vinegar, and 6 tablespoons ice water. Drizzle into the flour and mix just until it clumps. Add more water by the tablespoon if dry.
Divide into two discs, wrap, and refrigerate 45–60 minutes. This relaxes gluten and keeps the layers flaky. Yes, chilling is non-negotiable.
Stir preserves with cornstarch and zest or vanilla. Set aside.
Scramble eggs softly over low heat with salt and pepper until just set (slightly glossy). Stir in cheese and cooled, well-drained veggies/meat. Let cool to room temp; excess steam = soggy pockets.
On a lightly floured surface, roll one dough disc into a rectangle about 1/8-inch thick. Trim edges to square it off. Cut into rectangles roughly 3 x 4.5 inches (or Pop-Tart size, ~3 x 5 inches). Repeat with the second disc. You want an even number of rectangles.
Lay half the rectangles on a parchment-lined sheet. Brush edges with egg wash. Add 1–1.5 tablespoons filling to the center—don't overfill. Top with remaining dough rectangles and press edges to seal.
Use a fork to crimp edges firmly. Poke 3–4 small steam vents on top with a fork or paring knife. Brush tops with egg wash for shine.
Slide the tray into the fridge for 15 minutes (or freezer for 8). Cold dough bakes flaky and prevents leaks.
Heat oven to 375°F (190°C). Bake 18–24 minutes until golden brown. If baking two trays, rotate halfway. Cool 10 minutes before biting unless you enjoy lava-mouth.
Mix 1/2 cup powdered sugar with 1–2 teaspoons milk and a pinch of salt. Drizzle over cooled sweet pockets. For a lighter option, dust with powdered sugar or skip entirely—still great.