Toss the diced apples with the brown sugar and lemon juice in a bowl and let them sit for 30 minutes at room temperature. The sugar draws liquid out of the apples during this rest, and that liquid is what becomes the filling.

Remove from the heat and stir in the vanilla. Spread the filling out on a sheet pan to cool quickly, then refrigerate until completely cold and firm. This can be made a full day ahead and it must be cold before it touches the dough.

Combine the bread flour, starter, skyr, milk, sugar, eggs, and vanilla in the bowl of a stand mixer and mix on low until no dry flour remains. Cover and rest for 15 to 30 minutes.

Add the salt and baking powder and knead on medium low for about 5 minutes.

Add the softened butter a piece at a time, waiting until each addition disappears before adding the next. Continue kneading for 8 to 10 minutes until the dough is smooth, elastic, and pulls away from the sides of the bowl while still feeling slightly tacky.

Cover and bulk ferment at room temperature for 6 to 10 hours, until the dough has risen noticeably and feels airy and alive when pressed. In a 73 degree kitchen this takes about 6 to 7 hours. Cooler rooms take considerably longer, so watch the dough rather than the clock.

Turn the dough out onto a lightly floured surface and roll it into a rectangle roughly 12 inches wide by 18 inches deep, with the 12 inch edge closest to you. Roll away from you and toward you from the center rather than side to side, then roll from the center out to each corner on a diagonal to square up the shape.

Whip the softened butter until light and spreadable, then spread it across the entire surface of the dough, leaving a bare inch along the far edge.

Whisk the granulated sugar, cornstarch, cinnamon, and salt together, then sprinkle the mixture evenly over the butter. Do not stir it in and do not spread it. It needs to sit on top of the butter dry.

Dot the chilled apple filling across the surface with an offset spatula and spread it thin and even, working gently so you do not drag the sugar layer underneath. Take it to the edges on three sides and stop at the bare inch at the top.

Roll the dough up from the long edge closest to you, pulling back toward yourself every few inches to build tension. Pinch the seam closed along its whole length.

Cut the log into 12 rolls at roughly 1 inch each using unflavored dental floss. Slide the floss under the log, cross the ends over the top, and pull.

Arrange the rolls in a parchment lined 9x13 pan, three across and four down. Cover and proof at room temperature for 1 1/2 to 3 hours, until puffy, grown into each other, and jiggling slightly when the pan is shaken.

Combine the granulated sugar and water in a microwave safe bowl and microwave for 3 to 3 1/2 minutes, watching closely, until the mixture turns amber.

Carefully stir in the warmed heavy cream, which will bubble aggressively. Once the bubbling settles, stir in the butter, vanilla, and salt. Let it cool to room temperature or barely warm.

Whisk the powdered sugar and salt together, then whisk in 70 grams of the cooled caramel until completely smooth. Add the vanilla, then thin with the milk a few grams at a time until it pours in a steady ribbon rather than sitting in a lump.

Let the rolls cool until warm rather than hot, then pour the glaze over the pan. Drizzle the remaining caramel over the top in ribbons and finish with a pinch of flaky salt if you like.