In a small bowl, sprinkle the yeast over the warm milk. Stir and allow to sit until the yeast is frothy, about 10 minutes.
In the bowl of a stand mixer fitted with a dough attachment or a large bowl, combine the flour, sugar, cardamom, and salt.
Mix in the softened butter, egg, and yeast with milk to form a dough. Add a little more flour if too sticky (just enough to remove the stickiness- don't add too much or it will create tougher rolls) and a little more milk if too crumbly.
On a lightly floured surface, knead the dough until soft and smooth. Place the dough back in the bowl, cover with plastic wrap or a towel, and allow to rise until doubled, about 1 hour (this may take closer to 2 hours in cooler kitchens).
Preheat oven to 400˚F. Line two baking sheets with parchment or lightly grease.
On a lightly floured surface, divide the dough into 20 equal pieces, each about the size of a golf ball (45 grams/1.5 ounces). Roll each piece into a smooth ball and place on prepared baking sheets around 2 inches apart. Cover with dish towels and allow to rise until puffed, about 30 minutes.
Use a finger to make a depression in the center of each roll. Cut the chilled butter into 20 equal cubes. Place a cube in each depression.
In a small bowl, beat the egg and brush it over each roll. Pour 1/4 teaspoon of granulated sugar directly over each cube of butter in the bun. If desired, sprinkle pearl sugar around the edges.
Bake the voisilmäpullat in the preheated oven until golden, about 12 minutes.
Allow to cool on a wire rack and they are best served warm.