To form the buns:

Cup of Yum
1. In a small bowl, sprinkle the yeast over the lukewarm milk. Stir briefly to combine, then allow to sit at room temperature until frothy, about 10 minutes.
2. In the bowl of a stand mixer fitted with a dough hook or a large bowl, combine the flour, sugar, cardamom, and salt.
3. Mix in the softened butter, egg, and frothy yeast with milk to bring together the dough.
4. On a lightly floured surface, knead the dough until soft and smooth. After mixing, if the dough is still too crumbly and just won't come together, add a splash more lukewarm milk at a time. If just too sticky to handle, slowly add just a little more flour.
5. Place the dough back in the bowl, cover with plastic wrap or a towel, and allow to rise at room temperature until doubled, about 2 hours.

To assemble:

1. Preheat oven to 400˚F (200˚C). Line two rimmed baking sheets with parchment or lightly grease.
2. Divide the dough into pieces about the size of a golf ball (45 grams/1.5 ounces each), to make 18-20 in all.
3. Form each piece into a smooth ball and place on the prepared baking sheets about 2 inches (5 centimeters) apart. Cover with a large towel and allow to rise at room temperature until puffed, about 30 minutes.
4. In a small bowl, beat the egg until smooth. Brush the egg over each puffed bun.
5. Cut the butter into 5 separate tablespoon slices, then cut each tablespoon into 4 equal squares to make 20 squares in all.
6. Press each square of butter into the center of each bun.
7. Pour 1/4-1/2 teaspoon of granulated sugar directly over each cube of butter in the bun. If desired, sprinkle pearl sugar around the edges.
8. Bake the voisilmäpullat in the preheated oven until golden, about 12-14 minutes.
9. Allow to cool slightly on a wire rack and serve warm with milk or coffee.