Step 1: Make the pastry dough
In a large bowl, whisk together flour, sugar, and salt. Add cold butter cubes and use a pastry cutter, two forks, or your fingers to cut the butter into the flour until it resembles coarse crumbs. Add ice water, starting with 1/4 cup, and mix gently with a fork. Add more water, a tablespoon at a time, until the dough comes together but isn’t sticky. Divide the dough in half, form into two discs, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Step 2: Prepare the filling
If using jam or preserves, mix with a little cornstarch slurry to help it thicken while baking and prevent leakage. Stir well and set aside. If using chocolate spread or lemon curd, no prep is needed — just have it ready for assembling.

Step 3: Roll out and cut the dough
On a lightly floured surface, roll out one dough disc into a rectangle about 1/8-inch thick. Trim the edges to form a neat rectangle, then cut into even rectangles (about 3×4 inches). Repeat with the second dough disc. You should have 16 rectangles total, which will make 8 toaster pastries.

Step 4: Assemble the pastries
Place 8 rectangles on a parchment-lined baking sheet. Spoon about 1 to 1.5 tablespoons of filling into the center of each, leaving a 1/2-inch border. Place the remaining rectangles on top. Use a fork to crimp the edges all the way around and seal tightly. Poke a few holes in the top with a fork or skewer to allow steam to escape.

Step 5: Bake
Preheat your oven to 375°F (190°C). Chill the assembled pastries in the fridge for 10–15 minutes while the oven preheats to help them keep their shape. Bake for 20–22 minutes or until golden brown. Remove and let cool completely before glazing.

Step 6: Glaze and decorate
In a small bowl, whisk together powdered sugar, milk, and vanilla extract until smooth and pourable. Add more sugar for a thicker glaze or more milk to thin it out. Add food coloring if desired. Spoon or drizzle the glaze over cooled pastries and top with sprinkles or crushed freeze-dried fruit. Let set for at least 10 minutes before serving or storing.