Preheat the oven to 180C.
Warm the milk and saffron gently in a medium pan on low heat. Remove from the heat as soon as it starts steaming and set aside to steep for 10 minutes.
Add the butter into a 32cm x 24cm high sided baking dish and place into the oven for 5 minutes or until melted.
Put the cardamom and sugar into a large heatproof bowl and whisk in the eggs and yolks, followed by the milk, 300ml of the cream, orange zest and ¼ teaspoon of salt.
Toss the panettone through the melted butter in the baking dish and then carefully pour over the custard making sure everything gets an even soaking.
After 5 minutes, gently turn over any pieces that look dry so that the custard gets distributed evenly and then place in the oven and bake for 35 minutes until just set and golden brown on top.
Let it sit for 10 minutes, scatter over a little flaked sea salt and using a fine sieve, dust over some icing sugar.
Serve with the remaining 150ml cream on the side for pouring.