In a bowl, mix together the dough ingredients until crumbly. Gently shape the dough into a round, cover the bowl, and refrigerate for 1 hour.
For the filling
Mix the cheeses, egg, and potato, if using, in a bowl until smooth.
For the assembly
Preheat the oven to 350°F (177°C).
Divide the dough into 4 portions. Working with 1 portion dough at a time, turn it onto a lightly floured work surface and roll it to 1/8 inch thickness. Using a round cookie cutter or an upside-down glass that's 3 to 3 1/2 inches in diameter, cut out circles from each portion. You should have 24 to 30 circles total.
Place 1 teaspoon filling in the center of each circle, fold in half to form a semicircle, and pinch the edges to seal. (You may have a little filling left over after making all the borekitas.) Transfer the filled pastries to a baking sheet lined with parchment paper.
For the topping
Brush the top of each borekita with the lightly beaten egg and sprinkle with sesame seeds.
Bake until golden brown, 25 to 30 minutes.
Remove the borekitas from the oven, let cool to warm or room temperature, and succumb. (It turns out that the baked borekitas freeze well, too, allowing you to make them ahead and thaw them for a few hours on the kitchen counter.) Originally published August 4, 2014.