Warm milk in a small pot over medium heat until it’s warm to the touch but not yet simmering (110 degrees). Add 2 tablespoons sugar and the yeast, whisking to dissolve and break up any clumps. Let sit until it’s slightly foamy and starting to bubble, about 2 to 5 minutes. If it doesn't bubble, discard and start again with a new package of yeast.
Meanwhile, combine flour, the remaining 2 tablespoons sugar, the baking powder and the salt in a large bowl or in the bowl of a stand mixer fitted with the paddle attachment. Mix briefly to combine.
Add milk mixture, followed by the egg and, with a wooden spoon or the mixer on low, stir to combine, just to eliminate any obviously dry or wet spots. If using the mixer, you may need to scrape the dough off the paddle. Add melted butter and continue to stir until a shaggy lump of dough forms, again scraping down the paddle and helping things along with your hands if necessary to combine.
Turn dough out onto a lightly floured work surface, or keep in the mixing bowl and change to the dough hook attachment. Knead dough until it’s smooth, shiny and elastic, about 4 or 5 minutes.
Lightly grease a large bowl with nonstick spray and place dough in the bowl. Cover tightly with plastic wrap and place in a warm, draft-free area. Let sit until the dough has about doubled in size, about 2 hours.
Punch dough down to release any air that has built up. Cover tightly with plastic wrap, place in the refrigerator and chill the dough for at least an hour.
Once dough is chilled, turn it out onto a lightly floured work surface and roll to a 16 x 10-inch rectangle. The rolled dough will be about 1/4- to 1/3-inch thick.
Combine brown sugar, cinnamon and salt in a small bowl. Brush surface of the dough with half of the melted butter and sprinkle brown sugar mixture in an even layer, and sprinkle the mini chocolate chips on top, patting to make sure it’s sticking. Drizzle with remaining butter.
Starting at the end closest to you (the long side of the dough), roll dough up into a tight coil, pressing lightly as you roll to make sure there are no gaps between the dough and the filling.
Cut log into nine even pieces, about 1 3/4-inch thick. Lightly spray a 9 x 13-inch baking dish with nonstick spray and arrange dough in three rows of three pieces each, with the spiral facing upward and with space in between each piece. (At this point, the rolls can be covered tightly with plastic wrap and refrigerated overnight, if you'd like.)
Cover rolls lightly with plastic wrap and place in a warm, draft-free area until the rolls have puffed (they won't quite double, but they will be noticeably fluffier and closer to touching) and spring back slightly when pressed with your finger, about 1 hour. (If you refrigerated the rolls overnight, this may take 1 1/2 to 2 hours.)
Heat oven to 375 degrees. Place rolls in oven and bake until they are golden brown and fluffy and the sugar has started to bubble up around the edges, caramelizing on the sides of the buns, about 32 to 35 minutes.
Meanwhile, make the glaze: combine confectioners' sugar and milk and whisk until it is thick but can still be drizzled, like a thinner frosting. (If the glaze is still too thick, thin with more milk by the teaspoonful to get desired consistency.) Keep in mind the glaze is to be applied while buns are warm, and it will thin out upon contact, so it’s best to err on the thicker side.
Once rolls are out of the oven, drizzle with the powdered sugar glaze and let cool slightly in the pan before digging in.