Heat the oven and prepare the dish. Preheat the oven to 175°C (350°F). Grease a 9x13-inch baking dish and line the bottom with parchment paper. A light-colored metal dish gives the most even bake; a glass dish works too, just add 5 minutes to the bake time.

Whisk the eggs and sugar. In a large bowl, beat the eggs and sugar with an electric mixer on medium-high for 3–4 minutes, until the mixture is pale, thick and ribbony — it should fall from the whisk in a slow ribbon that holds its shape for a second. This is what makes the sponge light enough to soak up the milk.

Add the wet ingredients. With the mixer on low, add the milk, oil, vanilla and cardamom and mix until just combined. The mixture will thin out — that is normal.

Fold in the dry ingredients. Sift the flour, baking powder and salt over the bowl and fold gently with a spatula until no streaks of flour remain. Do not overmix — stop as soon as the flour disappears, or the cake will be tough instead of soft.

Bake the sponge. Pour the batter into the prepared dish and smooth the top. Bake for 28–30 minutes, until the top is golden and a toothpick inserted in the center comes out clean. Let the cake cool in the dish for 20 minutes, then cool completely — at least 1 hour — before soaking.

Make the saffron milk. While the cake cools, warm the whole milk, evaporated milk and condensed milk in a small saucepan over low heat — do not boil. Crumble in the saffron threads and stir in the rosewater if using. Let the milk sit off the heat for 10 minutes so the saffron releases its color and fragrance; the milk should turn a pale gold.

Soak the cake. Poke holes all over the cooled cake with a fork or a skewer, about 2 cm apart. Pour the saffron milk slowly and evenly over the cake, a little at a time, letting it sink in before adding more. Pouring slowly is the difference between an evenly moist cake and a soggy bottom.

Chill, top and serve. Cover the dish and chill for at least 2 hours, until the milk is fully absorbed. Just before serving, whip the cream with the powdered sugar until soft peaks form, spread it over the cake, and sprinkle with chopped pistachios and dried rose petals if you like. Slice and serve cold.