For dough: In the work bowl of a food processor, pulse flour and salt until combined. Add cold butter, and pulse until butter pieces are about the size of a quarter. With processor running, add ½ cup ice water in a slow, steady stream just until dough comes together. (Mixture may appear crumbly. It should be moist and hold together when pinched.) Add more ice water as needed.
Turn out dough, and divide in half. Press each half into a flat square. Wrap tightly in plastic wrap, and refrigerate for at least 1 hour or up to 3 days, or freeze for up to 3 months.
For filling: In a medium bowl, stir together all ingredients until combined.
Line a rimmed baking sheet with parchment paper.
In a small bowl, whisk together egg and 1 tablespoon water.
On a lightly floured surface, roll half of dough into a 13x11-inch rectangle. Place on prepared pan. Spread filling onto dough, leaving a 1-inch border on all sides. Brush border with egg wash.
On a lightly floured surface, roll remaining dough into a 13x11-inch rectangle. Carefully place dough on top of filling, gently pressing edges to adhere. Trim edges to straighten sides. Crimp edges with a fork. Freeze until firm, at least 20 minutes.
Position oven rack in bottom third of oven. Preheat oven to 400°.
Brush dough with egg wash. Using a paring knife, cut small vents every 1½ to 2 inches, without cutting into bottom layer of dough.
Bake for 10 minutes. Reduce oven temperature to 375°, and bake until golden brown, 15 to 20 minutes more. Let cool completely on pan.
Spread Vanilla Glaze onto cooled pastry. Top with sparkling sugar, if desired. Refrigerate in an airtight container for up to 3 days.
Recipe by Louisiana Cookin' at https://louisianacookin.com/toaster-pastry-king-cake/