Semolina flour gives this easy-to-make cake its pleasantly coarse texture and, with an assist from saffron, its lovely yellow hue.
Make the syrup: In a small saucepan, combine sugar, saffron, lemon juice, and 1/2 cup water. Bring to a boil, reduce to a simmer, and stir constantly until sugar dissolves.
Make the cake: Preheat oven to 350 degrees. Butter a 9-by-13-inch pan and line with parchment paper, leaving a 2-inch overhang on all sides; set aside.
Add saffron to milk and let steep 5 minutes. Whisk together coconut, semolina flour, baking powder, and salt in a large bowl until combined. In a separate large bowl, whisk together eggs, sugar, and oil until light and fluffy. Add milk mixture and yogurt; stir to combine. Fold in coconut mixture to incorporate.
Transfer batter to prepared pan and bake until cake is golden brown and springs back in center when gently touched, about 1 hour.
Transfer pan to a wire rack. Pour syrup over warm cake and sprinkle with pistachios. Let cake cool completely in pan on a wire rack. Using parchment overhang, remove cake from pan to a cutting board; cut into triangles using a serrated knife.