Preheat oven to 400˚F (200˚C). Place the rack in the top third of the oven, making sure there is space above for the rolls to rise. Line two rimmed baking sheets with parchment or lightly grease.
In a small bowl, whisk together the sugar and cinnamon.
On a lightly floured surface, roll the dough into a large, thin rectangle about 14 x 22 inches (35.5 x 56 centimeters).
Gently cover the top of the rectangle with the softened butter, taking care not to tear the dough. Keep a 1/2 inch (1.25 centimeter) clear border around the edges.
Cover the butter evenly with the cinnamon sugar mixture.
Tightly roll up the rectangle, long side to long side, pinching the seam well to seal.
Use a sharp knife to cut the dough into alternating diagonal slices to create individual triangular-shaped pieces with a 1 inch (2.5 centimeter) short side and 2 inch (5 centimeter) long side.
Place the rolls long side down on the prepared baking sheets at least 2 inches (5 centimeters) apart.
Use the side of your hand, finger, or two thumbs to press down in the center of each roll to fan out the spiraled edges.
Cover the baking sheets with towels and allow to rest at room temperature until puffed, about 30 minutes.
In a small bowl, beat the egg well, then brush gently over the puffed rolls, taking care to not let any excess drip around the bottom or pool in the center.
Cover the center of each roll with a sprinkling of the pearl sugar.
Bake the rolls in the preheated oven until golden, about 12 minutes. If baking both sheets at the same time, rotate at around 6 minutes.
Remove the Korvapuustit from the baking sheets and allow to cool on a wire rack or enjoy warm.