vegetarianMake the dough
Whisk the warm milk with the instant yeast, sugar, and instant espresso in a large bowl, then let it sit for 5 to 10 minutes until foamy. Whisk in the egg, egg yolk, and melted unsalted butter, then stir in the bread flour and salt until a shaggy dough forms. Knead on a lightly floured counter for 8 to 10 minutes, adding only a dusting more flour if the dough is very sticky, until it feels smooth, elastic, and slightly tacky.
Let it rise
Transfer the dough to a clean bowl, cover it, and let it rise in a warm spot for 60 to 90 minutes until doubled in size.
Build bottom layer
While the dough rises, spread the softened unsalted butter over the bottom of a 9x13-inch baking dish, then add the brown sugar, white sugar, heavy cream, vanilla, and a pinch of salt. Stir and smear everything together with a spatula until it forms an even, sticky layer that reaches the corners.
Mix the filling
In a medium bowl, mash the softened unsalted butter with the brown sugar, dark cocoa powder, instant espresso, cinnamon, and salt until you have a glossy, spreadable filling.
Roll and fill
Punch down the dough and turn it onto a lightly floured counter. Roll it into a rectangle about 12 by 18 inches, then spread the filling all the way to the edges, leaving a 1/2-inch strip clean along one long side. Roll it up tightly from the opposite long side into a neat log.
Slice the rolls
Use a sharp knife to cut the log into 8 equal rolls, wiping the blade between cuts if the filling starts to smudge.
Arrange in pan
Set the rolls cut-side up in the prepared dish, nestling them close together so they have room to puff and touch as they bake.
Preheat the oven
Preheat the oven to 350° F so it is fully hot when the rolls are ready.
Second rise
Cover the pan and let the rolls rise in a warm spot for 30 to 45 minutes until they look noticeably puffy and the gaps between them are filling in.
Bake the rolls
Bake for 25 to 30 minutes until the tops are deep golden and the centers no longer look doughy. If the tops brown too quickly, loosely tent the pan with foil for the last few minutes.
Whisk the icing
Whisk the mascarpone with the heavy cream, sugar, and vanilla until smooth, thick, and spoonable, about 1 to 2 minutes. Stop as soon as it comes together so the icing stays silky instead of loose.
Finish and serve
Let the rolls cool for 10 minutes, then spoon the mascarpone cream over the warm tops and finish with a light dusting of cocoa powder. Serve them warm so the icing melts into the swirls.
