Centre a rack in the oven and preheat to 300°F (150°C). Generously butter four 3/4-cup ramekins. Line a rimmed baking sheet with parchment paper.
Whisk together the sugar, cardamom, and saffron in a small bowl. **
I like to start by making sure the diameter of the apples will fit well within the ramekins. If they seem a bit large, you can trim the apples to fit snugly. Peel the apples and cut them in half across the equator; remove the cores with a melon baller. Using a mandoline (and a Kevlar glove** to keep your fingers safe), slice the apples into 1/8-inch thick slices. Keep the slices in order so that you can reassemble the apples in the ramekins.
Slice by slice, put back together each apple in the ramekin, brushing each slice with some melted butter and then sprinkling it with the sugar and spice mixture. After the final slice, top each apple with a small round of parchment paper (about the diameter of the ramekin). Place each ramekin on a 9-inch x 12-inch piece of aluminium foil. Wrap the foil around the ramekins, and up and over the apples, pressing down the apples a bit, and seal the foil tightly. Put the ramekins on the parchment-lined baking sheet and poke two holes in the foil on top of each with the tip of a sharp knife.
Bake for 2 hours. Remove from the oven and set aside to cool. (I made these ahead in the morning and let them sit at room temperature all day.)
To serve, warm the ramekins in a 200°F oven for 10 minutes to reactivate the buttery apple juices. Remove the foil and the small disk of parchment. Use a thin flexible spatula to loosen the sides and bottom, then turn out onto a serving plate. Some of the spice-infused butter will be left behind in each ramekin; drizzle it over the apples.