Using too much egg

Although the first pierogi dough recipes did not use eggs, most modern recipes do. The egg provides protein and fats that help the dough maintain its structure, but too much egg can result in a tough dough. If you use a tough, eggy dough, it will cook hard and taste bitter. If you suspect that you used too much egg in your dough, you can still fix it. Add a few drops of milk or water at a time until the dough softens. If your dough becomes too wet, add some flour.

It's difficult to add less than a whole egg to your pierogi dough if you think it needs more structure. Unfortunately, eggs are pre-portioned, which makes scaling down recipes more difficult. Because cooking is more forgiving than baking, it is perfectly acceptable to crack an egg into a bowl, whisk it, and measure out half an egg (or a quarter) as accurately as possible.