Step 1: Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan or line it with parchment paper.
Step 2: In a small bowl, soak the saffron threads in 1 tablespoon warm milk for about 10 minutes. This helps release their color and flavor.
Step 3: In a large mixing bowl, whisk together the yogurt and granulated sugar until well combined.
Step 4: Stir in the vegetable oil (or melted butter), eggs, and vanilla extract into the wet mixture until smooth.
Step 5: In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and ground cardamom.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. Be careful not to overmix.
Step 7: Fold in the soaked saffron (along with the milk it was soaked in) and the chopped nuts, if using.
Step 8: Pour the batter into the prepared cake pan, smoothing the top with a spatula.
Step 9: Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Step 10: While the cake is baking, prepare the optional saffron syrup. In a small saucepan, combine water, 1/4 cup sugar, and 1/4 teaspoon saffron threads. Bring to a boil, then reduce heat and simmer for about 5 minutes until slightly thickened. Remove from heat and let cool.
Step 11: Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Step 12: If using, drizzle the cooled saffron syrup over the cooled cake for added flavor and moisture.