In a large bowl thoroughly combine melted butter, milk, beaten eggs, salt, sugar, and optional seasonings, if using.
In a liquid measuring cup, combine yeast with 1/2 cup warm water (105-110 degress F) and ½ tsp. sugar; cover and allow to proof.
Add the proofed yeast mixture and some of the flour to the ingredients in the bowl with the butter-milk mixture. Add enough of the remaining flour to create a workable dough. Knead dough until smooth.
Place dough in a clean, lightly oiled bowl, cover with plastic wrap and allow to double in size, approximately 1hour.
Remove dough and place on a clean work surface. Pat down dough with your hands to create a rectangular shape that’s about ¾” thick.
Using a pastry wheel, pizza cutter, or sharp knife, cut dough into uniformly-sized squares, triangles, or diamond shapes, for even baking.
Preheat oven to 375°F.
Arrange the cut pieces on two baking trays. Be sure to leave room between each piece to allow for expansion during baking. Lightly beat one egg and 1 Tbsp. water in a small bowl to create the egg wash for the topping. Brush the surface of each piece with the egg wash. Garnish the tops with sesame seeds and/or nigella seeds, if desired. Place one tray on the center rack and bake for about 15 – 25 minutes (depending on the size of the pieces) or until golden brown. When the first tray is done, follow the same procedure for baking the second tray. Remove baked koolunja from the baking tray and place on wire racks to cool completely.
Store cooled pieces in a container with a tight-fitting lid.