Whisk the cornstarch and water together until all the cornstarch has dissolved. (I just use a fork to mix–very easy.) Place the berries, sugar, cornstarch mixture, and lemon juice into a small saucepan over medium heat. Using a wooden spoon or rubber spatula, stir the mixture as it cooks. Break up some of the berries as you stir. Bring it to a simmer and allow to simmer for 3 minutes, stirring constantly. Remove pan from the heat and cool completely. The mixture will thicken as it cools. Once cool, filling can be covered and refrigerated for up to 5 days before using. Makes 3/4 cup of filling.

Whisk the flour, sugar, and salt together in a large bowl. Place the cold and cubed butter on top. Gently toss the flour and butter together with your hands, and then briefly rub the butter into the flour to begin combining them, as you can see in the video tutorial below. You do not want to break down the butter too much in this step. This step is only possible if the butter is very cold because if the butter is warm, you’ll end up with paste. I do not recommend a food processor, pastry cutter, or mixer for this step because it will break down the butter too much.

At this point, the butter is still in large cubes/chunks. Begin adding the ice cold water 1 Tablespoon (15ml) at a time until dough forms 1 large shaggy clump in your bowl. Use your hands to toss the mixture together after you add each Tablespoon. (I usually start with 2 Tablespoons (30ml) of water before I begin tossing together.) You can use a spatula or spoon for tossing, but I really do recommend your hands so you get a good feel of the dough. As the dough begins to hydrate after about 4 Tablespoons (60ml) of water, you can start lightly squeezing or clumping the dough together with your hands to help bring it together. Mixture will still be very shaggy, as you can see in the video and photos below. If your dough feels sticky and wet before adding 6 Tablespoons of water, your butter was likely too warm—you can continue with the recipe, but the dough will not be as flaky.

Pour the shaggy clump of dough out onto a lightly floured work surface. There will still be large chunks of butter at this point and that’s a good thing. Begin patting the dough down with lightly floured hands until it’s 3/4 – 1 inch thick, about a 5×8 inch rectangle. Fold the dough into thirds as if you were folding a business letter. Use your hands to gently flatten and smooth out any cracks in your dough. Wrap it up tightly in plastic wrap, parchment paper, or aluminum foil, or place into any tightly sealed container.

Refrigerate dough for at least 2 hours and up to 24 hours.

Take the dough out of the refrigerator to begin the “rolling and folding” process. If the dough chilled for longer than about 3 hours, it’s likely very stiff so let it rest for about 5 minutes before you begin rolling. Lightly flour a work surface. The dough gets sticky, so make sure you have more flour nearby as you roll and fold. Use your hands to gently flatten the dough into a small square. Using a rolling pin, roll the dough into a 6 inch wide and 12 inch tall rectangle that’s 1/2 inch thick. The exact dimensions are not important, but the thickness is. As you roll, it’s best to flip the dough over once or twice to make sure it’s not sticking to your work surface. Lightly flour your work surface as needed. Fold the rectangle into thirds as if it were a business letter. (See photos and video tutorial.) Turn it clockwise or counter clockwise and roll it out into a 6×12 inch 1/2 inch thick rectangle again. Then, fold into thirds again. Turn it clockwise or counter clockwise. You’ll repeat rolling and folding 4 more times for a total of 6 times.

Wrap up/seal tightly and refrigerate for at least 15 minutes and up to 24 hours. You can also freeze the dough at this point. See freezing instructions.

Line two large baking sheets with parchment paper or silicone baking mats. Rimmed baking sheets are best because butter may leak from the dough as it bakes. If you don’t have rimmed baking sheets, when it’s time to preheat the oven, place another baking sheet on the oven rack below to catch any butter that may drip.

Take the dough out of the refrigerator to begin the shaping steps. Lightly flour a work surface. Using a rolling pin, roll dough into a 12-inch square. Using a pizza cutter or sharp knife, cut into 9 4-inch squares. Transfer squares to prepared baking sheets. (4 on one, 5 on the other.)

Spoon 1 heaping Tablespoon of filling into the center of each square. Fold one corner over the filling to make a triangle. If some chunky filling has seeped out, use a fork to push it back in if you can. Using a fork, crimp the edges of the turnovers together to seal. If the dough has become too warm and sticky to crimp, refrigerate the triangles for 10 minutes. You want to make sure you have a tightly sealed turnover to help prevent filling from spilling out in the oven.

Chill the shaped turnovers in the refrigerator, covered or uncovered, for at least 15 minutes and up to 1 hour (or freeze, see freezing instructions below). You can preheat the oven and make the egg wash (next steps) as they finish up chilling.

Preheat oven to 400°F (204°C).

Whisk the egg and milk together. Brush on each turnover. Cut 3 slits into the tops of each to allow steam to escape. If desired, sprinkle with coarse sugar.

Bake for 22-25 minutes or until golden brown on top and around the edges. Some butter may leak from the dough, that’s completely normal and expected. Feel free to remove the baking sheets from the oven halfway through baking and brush the dough with any of the leaking butter, then place back in the oven to finish baking. (That’s what I do!)

Remove baked turnovers from the oven. Cool for at least 5 minutes before icing/serving. Centers will be hot.

Drizzle with vanilla icing if desired.

Cover leftover iced or un-iced turnovers and store at room temperature for 1 day or in the refrigerator for up to 5 days. Or you can freeze them for up to 3 months. Thaw before serving. Before enjoying, feel free to reheat leftover iced or un-iced turnovers in the microwave for a few seconds until warmed.