In a large bowl, combine the sugar and salt with 1 cup warm water. Sprinkle the yeast on top and let stand until foamy, about 5 minutes. Add 3 tablespoons of the melted butter; stir to combine. Add the flour to the bowl and stir with a wooden spoon until a dough starts to come together.
Turn out the dough onto a lightly floured work surface and knead until smooth and elastic, about 5 minutes. Transfer to a clean, lightly oiled bowl and cover with plastic wrap. Let stand in a warm, draft-free place until the dough has almost doubled in size, about 1 hour.
Preheat the oven to 475 degrees F. Position racks in the upper and lower thirds of the oven. Lightly oil two baking sheets.
Cut the dough into 8 pieces. Roll each piece out into a 20-inch long rope and twist into a pretzel shape.
Fill a large, wide pot with about 3 inches of water. Add the baking soda and bring to a brisk simmer over medium-high heat. Boil the pretzels, two at a time, until puffed and shiny, about 2 minutes; transfer to the oiled baking sheets as you go.
Brush the boiled pretzels with the remaining 2 tablespoons butter and then sprinkle salt as desired. Bake until deeply browned, 12 to 15 minutes, switching the positions of the baking sheets halfway through.
While the pretzels are baking, warm the milk in a small saucepan over medium heat. Add the chocolate-hazelnut spread and whisk until combined.
Serve the pretzels warm with the chocolate-hazelnut dipping sauce.