In a small bowl combine the warm water, 1 tablespoon of the sugar, and the yeast. Stir to combine and then set aside to allow it to foam. Then add the remaining sugar, milk, and vegan sour cream. Mix well and set aside.
Pour the salt and flour into a food processor. Add the cubed butter and pulse in short bursts for several seconds. Use a spatula to scrape the sides of the bowl. Repeat short bursts until the mixture has combined and the butter resembles crumbles the size of peas. It should be 10 to 15 pulses total.
Next, pour the crumbly flour mixture into a bowl and drizzle the yeast mixture over the top. Stir the two together until just barely combined. You want to see large flecks of the butter showing through. This is what creates the best, flaky pastries. Tear off a piece of plastic wrap and lay it on the counter and then turn the dough on that piece of plastic wrap. Wrap up the dough and place it in the fridge to chill out for at least four hours or more.
Rolling and Folding. After the dough has thoroughly chilled, it's time for the next step in the process, referred to as "rolling and folding". Pour a light layer of flour over a work surface. Keep some extra flour nearby because the dough is sticky and you're going to be using your hands with this.
First Fold: Use a rolling pin to flatten the dough into the shape of a rectangle 15" long. Next, fold one end of the rectangle-shaped dough toward you and fold the other end over that (like you're folding a letter).
Second Fold: Repeat that process by rolling it out into a 15" rectangle and fold both ends over like a letter.
Third Fold: Repeat that process one more time for a total of 3.
Chill the Dough: Cover the dough with plastic wrap and place in the fridge to chill for at least 30 minutes but preferrably up to 4 hours. I have left the dough in the fridge for up to 4 days before and it's been fine.
Making Braids: Once the dough has chilled, remove it from the fridge and cut it in half. Return one-half of the dough to the plastic wrap and put it back in the fridge. With the remaining half, place it on a lightly floured surface and roll it out into the shape of a 12x8 rectangle.
Next, use a knife to slice off the two corners of the rectangle farthest away from you and cut in two small triangles on the other end. (See the post for visual guide)
Spread half of the cooled blueberry filling in the center of the dough before cutting slanted strips along both sides. Beginning at the bottom, "braid" the dough, sealing the piece at the bottom. Very carefully transfer the braid to a rimmed baking pan lined with parchment paper. Refrigerate for 10 to 15 minutes.
Repeat these steps with the second half of the dough. When you're done, place that braid on a baking pan covered with parchment paper as well.
Fold the bottom end up to "seal" the filling in. Place the second pan in the fridge as you preheat the oven.
At this point preheat the oven to 400°F/200°C.