In a large mixing bowl, combine the flour, instant yeast, and salt. Mix well to evenly distribute the dry ingredients.
Add olive oil and warm water to the dry mixture, then stir until a shaggy dough forms. Knead the dough on a lightly floured surface for about 8 minutes until smooth and elastic.
Place the dough into a clean bowl, cover with a damp cloth, and let it rise in a warm spot for approximately 1 hour, or until it doubles in size.
Preheat your oven to 220°C (430°F). Lightly flour your work surface and punch down the dough to release air. Divide it into small equal portions and roll each into a 15cm (6-inch) circle with a rolling pin.
Using a sharp knife, cut each circle into a bat shape, with wings and ears, creating a whimsical silhouette. Carefully transfer each shape onto a parchment-lined baking sheet.
Spread a spoonful of your chosen fillings on one half of each bat shape, leaving a small border around the edge to seal.
Fold the dough over the filling to form a semi-circle, then pinch the edges firmly to seal. Shape the wings and ears as desired, and use a knife to add wing details for a fun, textured look.
Mix the beaten egg and brush it evenly over the top of each calzone using a pastry brush. This will give a shiny, golden finish once baked.
Bake the calzones in the preheated oven for 15-20 minutes, or until they are golden brown and puffed up. The aroma of baked dough and melted cheese will fill your kitchen.
Remove from the oven, let cool for a few minutes, then serve warm, enjoying the crispy exterior and gooey interior with your favorite dipping sauce or side.