First, bring your butter and eggs to room temperature by placing them on the countertop for roughly 30 minutes before you start. To quicken things along and prep for later, slice the butter into roughly 1tbsp (14.2g) pats.
Next, whisk the milk and honey together in a sauce pot on your stove top. Turn the heat to low and bring the mixture to about (125°F or 51°C), then pour the milk and honey into your mixing bowl.
The temperature should come down once it hits a cold bowl. Use a thermometer to make sure the temperature has fallen between 110-115°F (43-46°C). Once you are in that window, add in the active dry yeast and let it rest for 10-15 minutes, so it can bloom.
While that's working, measure out the flour and salt and set aside. Also, very lightly butter a large bowl for proofing the dough later.
When the yeast has bloomed, turn on the mixer with the whisk attachment and add in the eggs. Once incorporated, add in salt and then flour a little at a time until it's about the viscosity of batter.
Switch out the whisk attachment for a dough hook and continue mixing in the flour until it comes away from the sides of the bowl; It may take upwards of 15 minutes. At this point you can begin to add the butter pieces one-at-a-time. Once one mixes into the dough fully, you can add the next until all of the butter has been added.
Continue mixing until the dough gets shiny and smooth and you can gently touch it without it immediately sticking to your fingers. It may need another 10 minutes of mixing to get to this stage. At any point, you can stop to do a windowpane test, to see if the dough is ready to to proof. Get your hands slightly wet and pull some of the dough up to see if it gets thin enough to see light through it without breaking. When you can do that, scrape the dough into the lightly buttered bowl and cover with cling wrap.
Set it on the counter for 1-2 hours or until it doubles in size. Then, gently (so as not to deflate the rise thus far) place it in the fridge to chill for at least 2 hours. 24 hours is best if you have the time.
At least an hour before taking the dough out of the refrigerator, bring the butter for the filling to room temperature. To prep the rest of the filling, mix the brown sugar, cinnamon, nutmeg, kosher salt in a bowl. Make sure to press out as many clumps of brown sugar as possible, so the filling is relatively sandy and uniform. Next, prepare a 9in x 13in (23cm x 33cm) baking pan by lightly buttering it or spraying it with non-stick spray. You can also add a layer of parchment down if you want extra non-stick action. The parchment will stick to the butter or spray and keep its shape, once in the pan.
When ready, scrape the chilled/cold-proofed dough onto a generously floured counter and roll it to a 14in x 18in (36cm x 46cm) rectangle, about ¼in (.6cm) thick. Orient the rectangle so left to right is the long measurement (length) and front to back is the short one (width). Spread the room temperature butter across the dough leaving only a ½in (1.3cm) strip along the back length of the rectangle. Then, sprinkle the sugar and spice mixture over the butter as uniformly as you can.
Once the filling is completed, take the edges of the front length of the rectangle and begin rolling from the outside edges in with gentle tucking motions. The key is to gently pick up small sections, turn, and place them down as if you are pushing the dough over itself. Keep tucking along the length, as you go, until all of the rectangle is rolled up into one cinnamon roll log.
From there, use a ruler to gently score or indent 1½in (3.8cm) segments in the 18in (46cm) log. There should be 12 rolls sectioned off in total. Cut a piece of thin twine or floss or take out a serrated knife and cut along each indentation. For the thread method, slide the thread underneath the log as close to the counter as possible until it is centered under one of your marks. Bring the thread up and then cross the strands over each other bringing each side to the opposite hand it started in. Next, pull tight and the thread will cut right through the roll and come out clean. As each roll is cut, place it into the prepared pan in a 3 x 4 row/column situation.
Cover and rest your rolls on the counter top for 30-45 minutes for a second proof. They will puff up to the point of touching. While the cinnamon rolls proof, turn on your oven and preheat it to 375°F (190°C).
Next, uncover and bake your brioche cinnamon rolls for 22-28 minutes. While they are in the oven, make the frosting. With a stand or hand-mixer, whip the cream cheese and mascarpone until they lighten in color considerably. Add in the salt, lemon zest, orange blossom water, and vanilla and whip for another 30 seconds before fully incorporating the powdered sugar (icing sugar) a spoonful at a time. When mixed, set the frosting aside until it's ready to use.
After 22 minutes, take your rolls out of the oven and check their doneness with a probe thermometer. The internal reading should be between 180-190°F (82-88°C). If you have hit this number and the centers of the cinnamon rolls don't squish down easily when you tap them (especially the middle rolls) then they are done. If not, return them to the oven for another 3-5 minutes.
Once out of the oven, spread the frosting over the rolls and let it melt into all of the nooks and crannies. Rest everything for 5-10 minutes before serving.