Mixing:

1. Combine the flour, salt and sugar in a large bowl.
2. Grate the frozen butter into the dry ingredients with a cheese grater. Using your finger tips or a pastry cutter, cut the butter into the dough.
3. Add the sourdough starter, eggs and water and mix gently with fork to incorporate, then fold with your hands just until dough comes together.
4. Transfer dough to floured surface and knead it  until there are no dry bits
5. Divide the dough in half. Shape each half into a rough 7.5 x 12.5cm (3 x 5″) rectangle
6. Wrap in plastic or waxed paper, and refrigerate for at least an hour or even better overnight.

Rolling and Making the Pop Tarts:

1. Place one piece of the dough on a generously floured work surface, and roll it into a rectangle about 0.3cm  (1/8″) thick.
2. It should be large enough that you can trim it to an even 22.8 x 30.5cm (9 x 12″).  Trim off the edges
3. Roll the 2nd piece of dough the same as the first.
4. Press the edge of a ruler into the dough you’ve just rolled, to gently score it into thirds lengthwise and width-wise.
5. Cut each piece of dough into thirds, then into thirds again.  7.5 x 10cm (3 x 4″) ,9 rectangles
6.  Beat the eggwash egg and brush it over the entire surface of the dough.  (Reserve the extra egg wash for baking).
7.  Place a heaping tablespoon of jam into the center of each  rectangle
8.  Place the second sheet of dough on top of the first.  Use your fingertips or a chopstick to press the dough firmly around each pocket of filling.
9.  Chill the pop tarts in the fridge or freezer before baking.

Baking:

1.  Preheat the oven to 350°F (176°C).
2.  Eggwash the pop tart and bake for 30-35 minutes.
3.  Cool the pop tarts on a resting rack before glazing.

Glazing:

1.  Mix the water or milk and icing sugar until combined. 
2.  Using a spoon or offset spatula, spread the glaze over the pop tarts and sprinkle with sprinkles before the glaze sets.