In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, and salt. Add the softened unsalted butter and use your hands to work it into the flour mixture until it resembles crumbs.
2. Activate the Yeast
In a separate bowl or jug, whisk the lukewarm whole milk with the instant yeast until fully dissolved.
3. Combine and Knead
Pour the yeast mixture into the flour mixture along with the whole eggs and egg yolk. Stir everything together using a wooden spoon or your hands until a rough dough forms. Transfer the dough to a lightly floured surface and knead for 8 to 10 minutes, until smooth and elastic.
4. First Rise
Shape the dough into a ball and place it into a greased bowl. Cover with plastic wrap and let it rise at room temperature for about 90 minutes, or until doubled in size.
5. Make the Filling
In a separate bowl, mix together the brown sugar, sugar, and cinnamon. Set aside.
6. Roll and Fill
Once the dough has risen, turn it out onto a lightly floured surface and gently flatten it into a large rectangle. Spread softened butter evenly over the surface, then sprinkle the cinnamon sugar filling across the dough. I did this in two.
7. Roll and Slice
Starting from the bottom edge, roll the dough up tightly into a log. Slice into 2-inch thick pieces. It is best to use floss if not a knife will do the job.
8. Second Rise
Place the rolls cut side up into a baking dish. Cover with plastic wrap and let them rise again at room temperature for 30 to 45 minutes.9. Bake Bake in a preheated 375°F (190°C) oven for about 20 minutes, until golden brown. Let cool in the pan for 15 minutes before serving with icing.