In a bowl, whisk flour, caster sugar, and salt.
Rub in the cold vegan block butter until you have sandy crumbs with pea-size bits.
Add ice water 1 tablespoon at a time, mixing gently, until the dough just holds together.
Press into a disc, wrap, and chill for 20 minutes.
Roll to about 3 mm thick and cut into squares (about 10 cm for standard size).
Spoon a small amount of strawberry jam into the center of each square.
Fold two opposite corners inward to overlap, then fold the bottom corner up to form the envelope seal.
Brush with dairy-free milk, sprinkle with caster sugar, and chill the tray for 10 minutes.
Bake at 200 C (390 F) for 18 to 22 minutes until golden. Cool 10 minutes and dust with powdered sugar.