In a large mixing bowl, whisk together the flour, sugar, and salt. Cut cold butter into small cubes and add to the bowl. Use your fingertips or a pastry cutter to cut butter into the flour until incorporated pea-sized.

Slowly drizzle in the cold milk and stir until a soft dough forms. The dough should easily come together and not feel sticky.

Transfer the dough to a lightly-floured surface. Divide in half. Flatten each half into 1-inch thick discs and tightly wrap with plastic wrap. Refrigerate for at least 2 hours.

While the dough is refrigerating, prepare the jam. In a saucepan, mix together the strawberries and sugar. Bring to a simmer, about 5 minutes. In a small bowl, whisk together 1 tbsp cornstarch and 1 tbsp water. Pour slurry into the saucepan and stir. It will immediately start to thicken. Add the lemon zest. Simmer until thickened and jam-like. Transfer to a small jar and cool completely in the fridge to further thicken. 

Remove one of the dough discs from the fridge. Place on top of a sheet of parchment paper. Place the disc in the center of one sheet and place another parchment sheet on top. Roll the dough out into an 8x6.5" rectangle. Cut off any rough edges. Transfer the rectangle with the parchment still attached, to the fridge.

Repeat previous step with the second dough disc. Chill both rectangles for 10 minutes.

Cut both dough rectangles into four, equal-sized small rectangles. Take 2 tbsp of strawberry jam and place in the center of 4 of the 8 rectangles, leaving a border along the edges. Brush egg wash or non-dairy milk on the edges. Place the other rectangle on top of the one with jam. Seal the pop tart by crimping the edges with a fork. Pierce the center of each sealed pop tart with a fork. Transfer sealed pop tarts to a lined baking sheet. Refrigerate for 15 minutes.

Preheat the oven to 350°F. Before baking, brush each pop tart with egg wash or non-dairy milk and sprinkle with granulated sugar. Bake for 32-35 min. until golden brown. Cool for 15 min. before drizzling with icing. 

Whisk together icing ingredients in a bowl until creamy. Spread icing on each pop tart and sprinkle with rainbow sprinkles. The icing will harden and set in about 10 minutes.