Boortsog has many regional variations across Central Asia (e.g., Kazakhstan's Bawïrsaq). In Mongolia, they are traditionally thicker and tougher, which allows for long storage.
During dough preparation, long and thorough kneading until the dough is perfectly smooth is essential for the correct texture.
They are traditionally fried in mutton fat, but a great flavor can also be achieved using deep vegetable oil or clarified butter (ghee).
Dissolve sugar and salt in the warm water/milk. Add the melted butter and egg (if using).
In a large bowl, mix the flour with the liquid mixture. Knead thoroughly until a stiff and dense, non-sticky dough is formed. This step is important: the dough should be kneaded until perfectly smooth, with no air bubbles.
Cover the dough and let it rest for 30 minutes.
Roll the dough out on a board to a thickness of 1 to 1.5 cm (0.4-0.6 in).
Cut the dough into rectangles or squares (typical dimensions are approx. 2x5 cm to 4x8 cm / 0.8x2 in to 1.6x3.2 in).
Traditional shape: make a longitudinal slit in the center of the rectangle and pull one end through the slit, creating a knot or twisted shape (for a nicer appearance).
In a large, deep pot or fryer, heat the oil to a temperature of 185 °C (365 °F).
Fry the Boortsog pieces in small batches (to avoid cooling the oil). Fry for 2-4 minutes until they are uniformly golden-brown on all sides.
Remove the fried Boortsog with a slotted spoon and drain on paper towels to remove excess fat.
Serve Boortsog warm or at room temperature. They are traditionally served with Mongolian milk tea, Suutei Tsai, into which they are dipped, but they are also excellent with honey, jam, or simply dusted with powdered sugar.