In a large bowl combine the flour, baking soda, and salt. Add the yogurt and mix with a spoon until a shaggy dough forms.

Turn the dough out onto a lightly floured surface and knead for 5 to 7 minutes until smooth, soft, and elastic. If it is very sticky, sprinkle with a little extra flour.

Divide the dough into 4 equal pieces and shape each into a ball. Cover with a kitchen towel and let rest for 5 to 10 minutes.

While the dough rests, place the grated, well-drained cheese in a bowl and gently mix it to loosen any clumps.

On a floured surface roll each dough ball into a circle about 1/4 inch thick. Place one quarter of the cheese in the center of each circle.

Bring the edges of the dough up over the cheese and pinch them together to form a sealed pouch, making sure there are no gaps.

Gently flatten each pouch with your hands, then roll it out carefully into a thick flatbread, keeping the cheese enclosed.

Preheat a dry nonstick skillet over medium heat for 2 to 3 minutes. Place one flatbread in the skillet and cook for 4 to 6 minutes, until the bottom is golden and puffed in spots.

Flip and cook for another 3 to 5 minutes, reducing the heat slightly if it browns too fast, until the second side is golden and the cheese inside is melted.

For the egg-topped version, make a small shallow well in the center during the last minutes of cooking, crack an egg into the well, cover the skillet, and cook 2 to 3 minutes until the white is set and the yolk is still soft.

Remove the hot chaczapuri from the skillet and brush the top lightly with softened butter, if using. Repeat with the remaining pieces, then serve warm.