In a food processor, add the gluten-free flour blend, xanthan gum (if using), salt and sugar and pulse to combine.
Add cold vegan butter pieces and pulse until the butter is the size of peas, with some larger pieces.
Remove from food processor and place the flour mixture into a bowl. Add in water one tablespoon at a time just until your able to pinch the dough together. You may not end up using all of the water.
Pat into a flat circle, cover with parchment paper or plastic wrap and chill in the refrigerator for about 20 minutes.
While the dough is chilling, make the strawberry filling. Add strawberry jam to a small saucepan and heat over medium heat until warm.
In a small bowl, mix together cornstarch and water and whisk until no clumps remain.
Add the cornstarch slurry to the jam and whisk until combined. Bring the jam to a boil and cook, while stirring, an additional 3 minutes. Remove from the heat and allow to cool completely.
Once the dough is chilled, roll out the dough into two equal rectangles about 8x11 inches each, about 1/4 inch in thickness. I like to place mine back into the freezer for 10 minutes for better handling.
Preheat oven to 350F. Place one rectangle of pastry dough on a baking sheet lined with parchment paper.
Spoon the strawberry filling onto the pastry and spread with a spatula, leaving about 1/2 inch border.
Carefully place the other rectangle of pastry dough on top of the strawberry filling. Gently press the edges closed with your fingers. Crimp all the way around with a fork.
Generously poke holes into the top of the pastry.
Bake at 350F for 40-45 minutes, until the edges of the crust are lightly browned. Remove from the oven and let cool completely.
While the pop tart is cooling, make the glaze. In a small bowl, combine powdered sugar, almond milk, and vanilla. Whisk until combined and no clumps remain. You want the glaze to be rather thick so it doesn't run off the pop tart but still spreadable, so add additional powdered sugar or almond milk to reach the desired consistency.
Once the pop tart is cooled, spread the glaze over the pop tart with a pastry knife. Sprinkle crushed freeze dried strawberries over the top.
Allow to dry completely before slicing and serving.