• Heat the oven to 400°F. Line a large baking sheet with parchment paper. Use a second sheet if needed to leave at least 1 inch between the pastries.
• Cook the apples. Melt 2 tablespoons butter in a large skillet over medium heat. Add the apple slices and cook, stirring occasionally, for about 5 minutes, until they begin to soften and release some juice.
  2 tablespoons butter, 4 large Granny Smith apples
• Thicken the filling. In a small bowl, whisk together ⅓ cup brown sugar, the cornstarch, and cinnamon. Stir into the apples and cook for another 3 to 5 minutes, until the apples are soft but still hold their shape. Remove from the heat and let cool to room temperature.
  ⅓ cup brown sugar, 1 teaspoon cornstarch, 1 teaspoon ground cinnamon
• Cut the pastry. Unfold both pastry sheets. Cut each sheet along its fold marks into three strips, then cut each strip in half. You’ll have 12 rectangles. Arrange them on the baking sheets with at least 1 inch between them.
  1 17.3 ounce box frozen puff pastry
• Make the borders. Using a paring knife, lightly score a line about ½ inch inside the edge of each rectangle. Don’t cut all the way through. Prick the pastry a few times with a fork inside each border.
• Add the filling. Divide the cooled apples among the rectangles, keeping the filling inside the scored borders.
• Make the crumbs. Use a fork to mix the softened butter, brown sugar, and flour until it forms small clumps. Sprinkle the crumb topping evenly over the apples, keeping the pastry borders clear.
  2 tablespoons butter, 3 tablespoons brown sugar, ¼ cup all-purpose flour
• Brush the edges. Whisk the egg with 1 tablespoon of water. Lightly brush the exposed pastry borders with the egg wash.

  Note: You won’t need all the egg wash. Discard the remainder.
  1 large egg, 1 tablespoon water
• Bake for about 20 minutes, until the pastry puffs and turns golden brown. Transfer to a rack and cool for 10 minutes.
• Drizzle. Stir 2 tablespoons of heavy cream into the powdered sugar. Add a little more cream as needed until it drizzles easily, then spoon it over the danishes.
  2-3 tablespoons heavy cream, ½ cup powdered sugar

Storage

• Storage and reheating: Keep the danishes in a covered container on the counter for up to two days. Reheat in a 300°F oven for a few minutes to help crisp the pastry, or microwave an individual danish for 20 seconds for a quick warm-up.