Dry the phyllo: Preheat the oven to 300°F (150°C). Separate the phyllo sheets, tear them into rough pieces, and spread them in a single layer on a large baking sheet. Bake for 10–15 minutes, tossing once or twice, until the phyllo is completely dry and crisp, but not browned. Let it cool slightly, then crumble it into smaller pieces - roughly 1-inch (2–3cm) pieces.
Prepare the syrup: In a small saucepan combine the sugar, water, orange juice, and cinnamon stick. Bring to a boil over medium-high heat, then simmer for 4–5 minutes. Remove from the heat and let the syrup cool completely.
Preheat the oven: Preheat the oven to 350°F (180°C). Grease an 8-inch (20cm) square baking dish or a similar-sized baking pan with oil. For other pan sizes, see the Notes below.
Make the batter: In a large bowl whisk together the yogurt and sugar until smooth. Add the eggs, orange zest, orange juice, vanilla extract, baking powder, and a pinch of salt, then whisk until combined. Add the sunflower oil and whisk until smooth.
Add the phyllo: Add the dried phyllo gradually, a handful at a time, folding well after each addition so the pieces remain separated and evenly coated. Let it sit for 5 minutes so the phyllo can absorb some of the batter.
Bake: Transfer the batter to the prepared pan and spread it into an even layer. Bake for 45–50 minutes, or until the cake is puffed, deeply golden brown, and crisp around the edges.
Add the syrup: As soon as the cake comes out of the oven, poke a few holes using a knife or a skewer, then slowly pour the cooled syrup evenly over the hot cake, including around the edges. Add it in 2–3 additions, allowing each addition to absorb before pouring over more.
Serve: Let the cake rest until all the syrup has soaked in. Slice and serve as is or with a scoop of vanilla ice cream.
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Notes
Pan sizes: I used a 10×6-inch (25 × 15cm) rectangular casserole dish for this recipe. However, you can also use any of the following:
11 × 7-inch (28 × 18cm) baking dish (the cake will be thinner and may bake a little faster)
Adjusting for different pan sizes: If you're using a different pan size, adjust the ingredients based on the baking surface area:
9-inch (23cm) square pan: Increase all ingredients by about 35%.
11 × 7-inch pan: Increase all ingredients by about 30%.
9 × 13-inch pan: Double the recipe.
9-inch (23cm) round pan: Increase the ingredients by about 15%.
Extra orange flavor: For an even more intense orange flavor, add the peel or a zest of 1 orange to the syrup while it's simmering. Remove it along with the cinnamon stick before pouring the syrup over the cake.
Hot cake, cold syrup: For the best absorption, pour completely cooled syrup over the cake immediately after it comes out of the oven. Adding hot syrup to a hot cake may make it overly soft.
Control the sweetness: Traditional Portokalopita is very syrupy. If you prefer a less sweet cake, you can use about 3/4 of the syrup instead of the full amount.
Storage: Store the cake covered in the refrigerator for up to 4–5 days. Let individual slices sit at room temperature for 15–20 minutes before serving for a softer texture and fuller orange flavor.