Combine the frozen blueberries and sugar in a medium saucepan over medium heat. Cook, stirring occasionally, until the berries break down completely and the mixture is bubbling and deep purple, about 8 minutes.

While the blueberries cook, whisk the cornstarch into the lemon juice in a small bowl until completely smooth with no dry streaks.

Once the compote is bubbling and the berries have fully broken down, slowly drizzle the slurry into the pot while whisking constantly. Cook for 2 more minutes until the compote thickens and a line drawn through it with a spoon holds for at least 3 seconds. Remove from heat, stir in the salt, and transfer to a bowl. Press plastic wrap directly against the surface. Cool to room temperature, then refrigerate until completely cold and firm, at least 2 hours.

Warm the skyr or Greek yogurt and milk together in the microwave for 30 to 45 seconds until slightly warm but not hot. If the mixture is too hot it can affect your starter.

In the bowl of your stand mixer, whisk together the starter, warmed skyr or yogurt and milk mixture, vanilla extract, and eggs until smooth, then mix in the softened butter. In a separate bowl whisk together the bread flour, baking powder, salt, sugar, and lemon zest.

Add the dry ingredients to the wet mixture and mix until the dough just comes together and there are no dry bits remaining. Cover the bowl with a towel and let it rest for 15 to 30 minutes. This rest gives the Greek yogurt time to hydrate the flour fully before the final knead, which is important because yogurt hydrates flour differently than regular liquid.

After the rest, knead the dough with the dough hook on medium speed for 5 to 6 minutes until smooth and elastic. The dough will feel softer than a lean sourdough, which is correct for an enriched dough.

Place the dough in a lightly oiled bowl and cover. Bulk ferment at room temperature for 8 to 10 hours until the dough has increased noticeably in volume, looks domed, and feels airy and jiggly when you shake the bowl. Watch the dough rather than the clock.

When the dough is finished proofing, whip the room temperature butter with a hand mixer until light and fluffy. Set aside.

Rub the lemon zest into the granulated sugar with your fingertips for about 60 seconds until it clumps and smells fragrant. Whisk in the cornstarch until evenly distributed. Set aside.

Turn the dough out onto a lightly floured surface and gently roll into a 12 by 18 inch rectangle with the 12 inch edge parallel to your body. If the dough springs back, let it rest 5 minutes and try again.

Spread the whipped butter evenly across the entire surface all the way to the edges.

Sprinkle the lemon cornstarch sugar evenly over the butter and press lightly with your palm so it adheres.

Pull the cold compote from the refrigerator. Working quickly with an offset spatula, spread it in a thin even layer over the sugar, leaving a 1 inch border clean along the top edge.

Starting from the edge closest to you, roll the dough upward and away from you into a tight log, pulling back gently as you go to create tension. The log will be approximately 12 inches long.

Using unflavored dental floss, trim any uneven ends, then cut into 12 equal rolls at 1 inch each.

Arrange the rolls cut side up in a parchment lined 9 by 13 pan. Cover and proof at room temperature for 3 to 4 hours until puffed and passing the poke test. Press gently with one finger and the dough should spring back slowly. Under proofed rolls will force the filling out during baking.

Preheat oven to 375 degrees Fahrenheit. Bake for 22 to 26 minutes until golden. The center of a middle roll should read 190 degrees Fahrenheit on an instant read thermometer.

Drizzle over warm rolls so it runs into the swirls and lets the blueberry color show through. Serve immediately.