Preheat the oven to 350°F. Line a baking sheet with parchment paper. 

In a large mixing bowl, whisk together flour, sugar, and salt. Cut in butter using your hands or a fork until they resemble pea-sized pieces. Slowly drizzle in milk and combine with a fork, then switch to your hands to form mixture into a soft dough ball. 

On a lightly floured surface, roll the dough into a 7x4.5" rectangle. Cut in half to create two equal-sized rectangles. Spread 2 tsp of jam into the center of one rectangle leaving room on the edges. Lightly brush the edges with non-dairy milk. Place the other dough rectangle on top and seal with the back of a fork (making slight imprints). Lightly brush the surface of the pop tart with milk. Sprinkle on a bit of granulated sugar. 

Bake pop tart for 20 minutes until light golden brown. Combine powdered sugar and non-dairy milk to create the icing. Smother with icing and top with rainbow sprinkles.