Prepare the potatoes by boiling cubed russet potatoes until tender when pierced with a fork; allow them to cool fully, then pass through a potato ricer and reserve. Combine bread flour, granulated sugar, sea salt, and instant dry yeast in the bowl of a stand mixer fitted with a dough hook; stir briefly to integrate the dry ingredients. Pour in the whole milk and crack in the egg, then knead on low speed for about 5 minutes until the mixture forms a rough dough that may appear slightly dry. Gradually incorporate the softened butter, adding it cube by cube while the mixer runs; continue kneading for approximately 2 minutes more until fully absorbed. Add the riced potatoes to the bowl and knead further at medium speed for 10–15 minutes, checking that the dough passes the windowpane test when stretched thinly. Shape the dough into a smooth ball and transfer to a lightly oiled bowl to rest for 45–60 minutes at room temperature, or until expanded by about 60%. Refrigerate for an additional hour or until the dough has doubled in volume. Turn the dough out onto a floured work surface, divide evenly into 9 portions, and weigh them for uniformity. Form each portion into a smooth ball, then let them relax for 5 minutes. Flatten each ball into a rectangle roughly 4 inches (10 cm) in length; beginning from a long edge, roll each piece up tightly into a roulade. If the dough resists shaping, cover and allow it to rest briefly. Extend each roulade into strands about 11 inches (27 cm) long, lightly flouring them and brushing away any excess. Working one at a time, tie each strand into a knot. Arrange the knots within a 9×9-inch baking pan, cover, and allow to proof for 30–45 minutes until puffed and slightly jiggly when the pan is shaken gently. If desired, dust the tops lightly with flour before baking. Place the pan in a preheated oven at 350°F/180°C and bake for 15–20 minutes, ensuring the internal temperature reaches 195°F/90°C. Remove from the pan and let cool on a wire rack for at least 20 minutes prior to servin