Fill a 2-cup measuring cup with ice. Top measuring cup off with water (at least ½ cup). Set aside.

Cube butter into ½-inch to 1-inch pieces with varying rough chops. Toss together flour and salt in a large bowl.

Toss butter cubes into flour mixture, and separate cubes with your fingers before beginning to work together. Work in butter using your hands, making smears and buttery flakes. Do not use a machine or a pastry cutter; using your hands will help you discern the temperature and feel of the flour. You are looking to incorporate all the butter into the flour without overworking. Every piece of butter should be smeared with some flour before you begin to add the ice water; there should be no untouched pieces of butter. Doing this deftly is key; if the butter starts to feel too warm, use your refrigerator as a tool—place bowl in freezer or refrigerator at any time to cool off. Preferably, though, you will be able to work the butter in quickly without too much fuss.

To incorporate the ice water, start by drizzling about 3 tablespoons water (do not add ice) into flour mixture. Using your hands slightly cupped as paddles (do not use your fingers or do any kneading), toss the water into the flour until it is fully absorbed. Add additional water, 1 tablespoon at a time, paddling with hands between each addition, until mixture begins to form a shaggy dough. You will likely use about ¼ cup to ⅓ cup water total, depending on the humidity. The dough should feel moist but not wet or sticky. Turn dough out onto a clean work surface. Give dough a few strong kneads to work it all together (about 4 turns). Your dough should feel tacky and supple but not sticky.

Divide dough evenly into 2 pieces (about 12 ounces each). Form each piece into a flat disk and wrap individually in plastic wrap. Chill in refrigerator at least 1-hour or up to 3-days.

While dough is resting, heat a medium sauce pan over medium heat.

Add all ingredients and bring to a bare simmer. Reduce heat and cook for 12 to 15 minutes, stirring occasionally until blueberries have released their liquid and the sauce is smooth.

Remove from heat allow to cool.

Add egg whites to bowl of a stand mixer, whisk until frothy.

Add ¼ Cup of powdered sugar and mix on low until combined.

Slowly add the rest of the powdered sugar on medium-low speed, scraping down the sides until completely incorporated and smooth.

Increase the speed to high and whisk until soft peaks form, about 5 minutes.

Slowly add the lemon juice one tablespoon at a time until the desired consistency is reached. This should be fairly thick and hold its shape, yet still be slightly runny. (The blog post shows a picture of the consistency you're looking for.)

Combine beaten eggs and water in a small bowl. Set aside for assembly and baking.

Preheat an oven to 375°F.

Remove one of the pie dough disks from fridge and roll into a 10-inch tall by 15-inch wide rectangle with a floured rolling pin. Slice into 4 or 5 equal rectangles.

Brush a small amount of egg wash around the perimeter of each poptart.

Fold the top half over the filling and crimp the edges of the dough with your fingers or a fork. Repeat with remaining poptarts.

Brush each poptart with egg wash, and transfer to a baking sheet. Cut a small hole in the center of each poptart to allow steam to escape. (This will help to keep these from breaking open and leaking while they bake.)

Bake for 20 to 25-minutes until golden brown. Remove from oven and transfer to a wire rack to completely cool, approximately 1-hour.