Combine milk, yeast and 1 teaspoon sugar in a medium bowl; set in a warm spot and let sit until foamy, about 10 minutes.
In the bowl of a stand mixer, fitted with the dough hook, combine remaining sugar, milk and yeast mixture, flour, salt, eggs and butter; mix on medium until it dough starts to form. Knead on high 8 to 10 minutes, until smooth and somewhat soft. Transfer dough to a well-oiled bowl and cover with plastic wrap and a damp kitchen towel. Let rest in a warm spot until dough doubles in size, about 1 1/2 hours.
Melt butter in a large skillet over medium-high heat. Add ground sirloin and cook, breaking it up with a spoon, until browned. Add onion and garlic, cook 2 to 3 minutes. Stir in taco seasoning, tomato paste and 3 tablespoons water. Cook until the water evaporates. Remove from heat and let cool completely.
Punch dough down and transfer to a lightly floured work surface. Roll out into a large rectangle about ¼-inch thick. Spread the dough with the black beans and then sprinkle with the cooled beef and cheese. Roll into a tight log, length wise, and then cut into 12 even pieces. Place on a greased 9x13-inch baking pan. Cover loosely with plastic wrap and a damp kitchen towel. Allow to rest in a warm spot until doubled in size, about 1 hour.
Preheat oven to 375?F. Bake rolls until puffed and golden brown, about 30-35 minutes. Remove from oven and let cool slightly.
Meanwhile, to prepare the sauce, melt butter in a medium sauce pan over medium heat. Whisk in flour and cook 30 seconds. Slowly whisk in milk and cook until mixture thickens. Season with taco seasoning. Remove from heat and stir in cheddar cheese until smooth. Spoon queso over the warm buns and then top with diced tomato and avocado, fresh cilantro and crumbled queso fresco.