Preheat the oven to 175°C/fan 160°C. Butter a 22cm tin and line its bottom with baking paper.
Melt the butter in a pan set over medium heat, and add the saffron, stirring well.
Off the heat, mix in the white chocolate, and stir using a silicon spatula until fully melted.
Now whisk in the sugars, salt, and the eggs, one at a time, mixing well after each addition.
Add the flour, and mix until just combined.
Pour the batter in the prepared tin, and bake for 30 minutes - the edges should be set, while the middle remains wobbly.
Let the cake cool completely in the fridge for at least 4 hours, preferably overnight, before serving.
Serve the cake in thin wedges, with a generous spoonful of custard and sliced clémentines, or perhaps with loosely-whipped cream and fresh raspberries. Even sometimes, only a light dusting of icing sugar is enough.