Using a hand mixer, beat butter and tvorog until smooth.
Using a wooden spoon, mix in the flour and baking powder with the butter mixture until it's soft and tender. The dough should not stick to your hands and will be very soft. Add more flour to the dough if necessary as you work it all together. You can refrigerate for a few hours to firm up the dough, as it is soft, but I'm impatient and just start right away!
On a lightly floured surface, roll out the dough approx ¼" thick.
Cut out round circles from the dough. Put sugar on a plate and press only one side of the circle into the sugar. Press firmly so the sugar sticks to the dough, then fold circle in half. The sugar will always be folded together, inside of cookie, leaving the outside you are handling sugar free.
You now have a semi-circle piece of dough. Press firmly one side into the sugar and again fold the sugar into itself.
You now have a quarter circle. Press one side into the sugar and place the sugar side facing up on a baking sheet lined with parchment paper.
Bake for about 15 minutes at 180°C/350°F until they are puffed up and golden brown.