Preheat the Oven: Preheat the oven to 375°F (190°C).
Prepare the Pan: Grease a 12-inch (30.5 cm) baking pan or springform pan thoroughly with some of the melted butter.
Make the Cheese Filling: In a large bowl, whisk the eggs and yogurt together until the mixture is smooth.
Add the Cheese: Crumble the Sirene or Feta cheese into the yogurt mixture, then add the baking soda and whisk until everything is evenly combined.
Prepare the Phyllo: Carefully unroll the thawed phyllo sheets and keep them covered with a towel while working to prevent them from drying out.
Butter a Sheet: Place one phyllo sheet on your work surface and gently brush it with melted butter.
Add the Filling: Spread or place a thin line of the cheese-yogurt mixture along the long edge of the buttered phyllo sheet closest to you.
Roll the Phyllo: Tightly roll the phyllo sheet toward the opposite long edge, enclosing the filling inside.
Arrange the First Roll: Place the rolled phyllo seam-side down around the outer edge of the prepared baking pan.
Continue Rolling: Repeat the buttering, filling, and rolling process with the remaining phyllo sheets, arranging each roll around the pan and gradually working inward toward the center to form a coil.
Butter the Top: Brush the assembled Banitsa generously with the remaining melted butter.
Bake the Banitsa: Place the pan in the preheated oven and bake until the top is golden brown, about 45 minutes to 1 hour.
Cool Before Serving: Remove the baked Banitsa from the oven, cover the dish with a towel, and let it cool at room temperature for 10–20 minutes.
Serve: Serve the Banitsa with additional yogurt, or enjoy it alongside boza or ayran.