Preheat the oven to 400°F. 

Mix the cheeses and yogurt together in a large bowl. Add the eggs and baking soda. The baking soda should begin bubbling immediately. Mix well.

Cut the phyllo dough into 6-inch strips. Spread a little melted butter onto one side of each strip. Place a small amount of cheese filling onto each dough strip. Add spinach if desired. Gently roll the strips closed and place onto baking paper.

Coat the bourekas with melted butter and bake in the oven for about 20 minutes until they are golden and crispy