The night before, make your dough. Melt butter and stir together with the milk.*
Add eggs, starter, and sugar to a stand mixer fitted with a paddle attachment. Mix to combine.
With the machine running, add milk & butter mixture.
Switch to dough hook and add 600 g flour and salt. Continue mixing until a very very sticky, rough dough forms, scraping the bowl as needed.
Cover and let rest 30 min.
After the dough has rested, knead on high for 7 min until a smooth, soft, slightly tacky dough forms, scraping down the dough hook as needed. You should have a beautiful and elastic dough. If it's too tight, mix in a bit of water until it becomes supple. If it's still too sticky and not coming together too well, add a bit more flour.
Transfer the dough to a large oiled bowl, cover, and let rise in a warm place for 8-12 hrs.*
Once your dough has doubled in size, line 1 9x13 in pan and 1 8 in pan with parchment paper, and butter the surface. Empty the dough unto your work surface and divide in half.
Roll and pat each half into roughly a 16x12 in rectangle (I always measure) but make sure the rectangle isn't too thin; you want it to be 1/4 in thick or so
Spread 1/2 cup butter, 1/2 cup brown sugar, and 1/2 tbsp cinnamon unto your dough, and sprinkle the top with 1 diced apple.
Starting on the long side of the dough, roll it up tightly yet gently, coaxing the dough along as you go. You should end seam side down. Roll the seam up and pinch it tightly to seal before flipping it back over.
Using a serrated knife, cut the log into rolls about 1 in thick. Place in pan, making sure they touch and repeat with remaining dough.
Cover the rolls and let rise 1-2 hrs until very puffy. They don't have to be totally doubled in size.
Preheat oven to 375 F and bake for 15-25 min until risen and golden. Don't overbake.
Let cool for a bit and then very generously slather with your frosting. Rolls are best enjoyed warm. I pop them in the microwave for a bit when I want to warm them again.