“What are you doing now?”

B enjoys roasting me about my many kitchen experiments. It’s partly cultural weirdness — the things I do in the kitchen on even an ordinary day don’t always resemble what he grew up watching his mother do — and partly because, over the past few months, I really have scrambled out on a few culinary limbs. This cake was a source of endless wonder for him, for a few days, mainly I’m guessing because of the smells.

For a couple of days, while I experimented, the kitchen was filled with them, as oranges in sugar water with Madagascan vanilla bean bubbled away on the stove and dried roses and cardamom steeped in bowls full of hot water. He’s already adjusted to my strange demands that he save all his citrus peels so I can dry them for desserts and teas, but this was the first time he saw me pick up a piece of candied peel and pop it straight into my mouth. By the time the package of dried rose petals arrived, he’d stopped asking questions.

When we went home for Seollal (the Korean lunar new year celebration), I picked out a huge box of hallabong oranges as our gift for his family. The woman in the shop dutifully wrapped them in a soft-gold bojagi — a Korean wrapping cloth that doubles as covering for a gift and a handy way to carry packages. After dinner on the night of our arrival, my mother-in-law came out of the kitchen with a huge tray of fruit, including the hallabong oranges. B picked one up, dug his fingers into the thick, leathery skin to remove the peel and gently pried it apart. When he bit into the first slice, he immediately sucked in his face. “It’s too early in the season,” I said. “In February and March they’ll be better.”

Hallabong oranges are more commonly known by their Japanese name — dekopon. That may be because everything is more commonly known by its Japanese name, or it may be because Japan is actually where the oranges were developed, in the early ’70s. Pocked with deep dimples and sometimes wrinkly as a hound dog, hallabong oranges are easiest to spot because of the little mound near their stem. They’re considered a luxury fruit in Korea (a country I love for many reasons, not least of which being that fruit is treated as a luxury) and are often sold with the stem and leaves still attached.

Their Korean name comes from Mount Halla-san, on Jeju-do Island, located off the south coast of the peninsula. Jeju-do is where the fruit is mainly grown within Korea, as the climate there is better suited to citrus fruit.

B’s puckered face at Seollal wasn’t an overreaction. Hallabong oranges have an unusually high content of citric acid, which can be balanced beautifully by their characteristic sweetness, if you are patient enough. In Japan, the fruit are held from market for about a month after harvest to allow this change to happen — as the oranges become slightly overripe, the sweetness increases and the citric acid dissipates. It also helps if you catch the oranges near the end of their season, in March or April, but if you happen to buy a batch that’s overly bitter, don’t be afraid to leave them for a couple of weeks. They will taste better — I promise.

B’s mom, I guess remembering what I said when we visited, sent us six kilos of hallabong oranges in mid-February. I immediately set a few aside and gave B strict orders not to touch them, with the promise of something delicious in return for his patience.

I had two weeks of walking back and forth past the oranges in my kitchen to dream up a plan for them. This cake (along with the chocolates I’ll post next week) was well worth the wait.

If you don’t have access to hallabong, ordinary oranges will work just fine in this recipe, but the kicky tang of these Asian cultivars gives the cake another layer of flavor. Their seedless centers can make them a little tricky to slice thinly, so use a sharp knife and don’t feel bad if they’re not aesthetically ideal — mine weren’t, either.

If you want to make some hallabong chocolates next week, peel an extra orange and thinly slice the peel. Double the water, sugar, cardamom and vanilla in the orange syrup recipe and toss in the extra peel. Keep the leftover syrup in the fridge and watch this space for the recipe.