In a food processor, pulse flour and salt until well combined.

Add cold, cubed butter and pulse until mixture resembles very small pea sized pieces or coarse meal.

Add apple cider vinegar to food processor with ½ cup ice water. Pulse mixture until a very shaggy dough forms.

Open the top of the food processor and feel the dough. If it is too dry, add another tablespoon or 2 of the ice water and pulse briefly to bring dough together into a more cohesive mass.

Place mass of dough into a large bowl and press into a ball.

Place dough onto a large sheet of plastic wrap. Wrap dough and press it into a large rectangular disk.

Chill in the refrigerator for approximately 1-2 hours or overnight.

Place puréed and sieved berries in a heavy medium saucepan with the sugar and blend well. Taste for your desired level of sweetness. Add another teaspoon or two of sugar, if needed.

Add lemon juice and mix well.

In a small cup, whisk together the cornstarch and water until cornstarch is dissolved.

Whisk cornstarch mixture into blackberry purée and bring the mixture to a simmer on medium high heat.

Allow the mixture to thicken and boil for approximately 20-30 seconds.

Remove the saucepan from the heat.

Pour filling into a bowl, cover and chill in the refrigerator until very cold. Alternatively, pour filling onto a parchment covered sheet pan with sides to chill and thicken faster.

Remove dough from the refrigerator.

On a lightly floured work surface, using a dough cutter, divide the dough in half.

Cover 2 large sheet pans with parchment paper and set aside.

Working with half of the dough at a time, place the remaining half, wrapped back into the refrigerator to stay cold.

Roll half of the dough into a 16 by 11 inch rectangle approximately ¼ to 1/8 inch thick. Using a ruler and pizza wheel trim the edges so that the rectangle now measures 15 by 10 inches.

Using a pizza wheel and ruler, cut the dough in half lengthwise.

Cut the dough crosswise every 3 inches creating 10 rectangles approximately 5 by 3 inches.

Gently lift the rectangles and place them on the prepared sheet pans, placing 5 rectangles on each sheet pan spacing them slightly apart. Set aside.

These 10 rectangles will be the bottoms of the pop tarts.

Repeat rolling out second-half of dough just like you did in steps 5 through 7. Leave the remaining 10 rectangles on the work surface until you are ready for them.

Using a pastry brush, lightly egg wash all along the outside edges of each rectangle of dough on the two sheet pans.

Place one heaping tablespoon of blackberry filling in the center of each rectangle.

Using a small offset spatula, spread the filling out and stop just before you get to the edges.

Gently place the rectangles that will be used as tops on top of each of the filled rectangular bottoms.

Using a fork, crimp all around the four edges to seal in filling.

Preheat oven to 400 degrees Fahrenheit.

Remove sheet pans of pop tarts from the refrigerator and, using a pastry brush, lightly egg wash the tops of each one.

In a large bowl, whisk together confectioner sugar, cream, vanilla and 1 tablespoon reserved blackberry filling.

Add a little more cream to get a spreadable icing, if it is too thick.

Allow glaze to firm up.