Step 1: Warm the milk to 105-115°F (40-46°C). Pour into a large mixing bowl or the bowl of a stand mixer. Sprinkle in the active dry yeast and 1 teaspoon of granulated sugar. Let it sit for 5-10 minutes until foamy, indicating the yeast is active.
Step 2: Add the remaining ¼ cup granulated sugar, melted butter, and eggs to the yeast mixture. Whisk until well combined.
Step 3: Gradually add 4 ½ cups of all-purpose flour and salt. Mix with a spoon or the dough hook of a stand mixer on low speed until a shaggy dough forms. Increase speed to medium-low and knead for 5-7 minutes, or until the dough is smooth, elastic, and pulls away from the sides of the bowl. If too sticky, add up to ½ cup more flour, 1 tablespoon at a time. It should be slightly sticky but manageable.
Step 4: Lightly grease a clean large bowl with cooking spray or a little oil. Transfer the dough to the bowl, turning it once to coat. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm place for 1-1.5 hours, or until doubled in size.
Step 5: Once the dough has doubled, gently punch it down to release the air. Turn it out onto a lightly floured surface. Roll the dough into a large rectangle, approximately 18×12 inches (45×30 cm).
Step 6: In a small bowl, combine the ½ cup softened butter, brown sugar, cocoa powder, and 1 tablespoon instant espresso powder for the filling. Mix until a smooth paste forms. Evenly spread this filling mixture over the entire surface of the dough rectangle, leaving a small border (about ½ inch) on one of the longer edges.
Step 7: Starting from the longer edge opposite the border, tightly roll the dough into a log. Pinch the seam closed to seal.
Step 8: Using a sharp, un-serrated knife or unflavored dental floss, cut the log into 12 even slices (about 1.5 inches thick each). Arrange the sliced rolls in a lightly greased 9×13 inch baking dish, leaving a little space between each roll.
Step 9: Cover the baking dish with plastic wrap or a clean towel and let the rolls rise again in a warm place for another 30-45 minutes, or until noticeably puffy and almost touching each other.
Step 10: While the rolls are rising, preheat your oven to 375°F (190°C).
Step 11: Bake the rolls for 25-30 minutes, or until golden brown on top and cooked through. If they start browning too quickly, you can loosely tent them with aluminum foil.
Step 12: While the rolls are baking, prepare the espresso drizzle (if using) by stirring 1 tablespoon instant espresso powder into 2 tablespoons hot water until dissolved.
Step 13: For the frosting, in a medium bowl, beat together the softened cream cheese and mascarpone until smooth and creamy. Add the powdered sugar, vanilla extract, and heavy cream. Beat on medium speed until light, fluffy, and smooth. Be careful not to overbeat.
Step 14: Once the rolls are out of the oven, let them cool for 5-10 minutes in the pan. If using the espresso drizzle, gently brush or spoon it over the warm rolls. Then, generously spread the mascarpone cream cheese frosting over the warm rolls. Serve immediately.