Yaki-Imo Sweet Potato Casserole with Toasted Mochi & Miso Brown Butter

Why It Works

The magic of yaki-imo is enzymatic. Sweet potatoes contain beta-amylase, which converts starch into maltose, and it's most active between roughly 135°F and 170°F. Street vendors roast on hot stones for a long time, holding the potatoes in that zone and making them dramatically sweeter and more custardy. A long 300°F roast does the same thing at home before the 400°F finish caramelizes the sugars. Because the potatoes get so sweet on their own, the casserole needs little added sugar. White miso and browned butter add savory, nutty depth that keeps it from being cloying. Kiri mochi is the perfect marshmallow stand-in: it puffs, blisters, and turns chewy-crisp in a hot oven, echoing the toasted mochi of Japanese winter. Black sesame and kuromitsu bring the roasty, molasses notes of Japanese autumn sweets.

Altitude note: roasting times hold at Front Range elevation, but check that the potatoes are fully soft in the center before mashing.

Inspired by the yaki-imo trucks and the Imo Nokke sweet potato soft-serve from fall in Japan.

Pairings

• Roast turkey, or a miso-glazed turkey breast
• Hojicha, genmaicha, or a warm junmai sake
• Leftovers: shape into balls and fry as sweet potato croquettes, or blend into a soft-serve base

Yield

8–10 servings (one 9" × 13" dish)

Time

2 hours 45 minutes (mostly hands-off roasting)

Equipment

• Sheet pan + wire rack or foil
• 9" × 13" baking dish
• Light-colored saucepan (for browning butter)
• Potato ricer, food mill, or stand mixer
• Instant-read thermometer (optional)

Ingredients

• Yaki-imo base
• 2 kg Japanese sweet potatoes (satsumaimo: Murasaki, Beni Haruka, or Okinawan purple-skinned with white flesh). Garnet yams work but are less chestnutty.
• 90 g unsalted butter
• 40 g shiro (white) miso
• 120 ml heavy cream, warm
• 30 g mirin
• 15 g honey or kuromitsu (optional; yaki-imo is naturally very sweet)
• Salt, to taste
• Toasted mochi topping
• 200 g kiri mochi (shelf-stable rice cake blocks), cut into 1/2" cubes
• 15 g butter, melted
• Black sesame crunch
• 40 g panko
• 25 g black sesame seeds
• 20 g light brown sugar
• 25 g butter, melted
• Pinch of salt
• To finish
• Kuromitsu (black sugar syrup), or dark honey or maple syrup
• Flaky salt
• Optional: a pinch of kinako (roasted soybean flour) or a few curls of candied yuzu peel

Instructions

Roast yaki-imo style (up to 2 days ahead)

• Scrub the sweet potatoes and leave them whole and unpeeled. Wrap each loosely in foil (or set on a rack) on a sheet pan.
• Roast at 300°F for 90 minutes, then raise to 400°F for 30–40 minutes, until completely soft, oozing dark syrup, and the skins pull away.
• Cool slightly, then scoop out the flesh. Save a few bits of caramelized skin to chop into the topping if you like.

Miso brown butter

• Brown the 90 g butter in a light-colored pan over medium, swirling, until it smells nutty and the solids are hazelnut-brown. Take it off the heat and whisk in the miso until smooth.

Base

• Rice or mash the warm sweet potato. Beat in the miso brown butter, warm cream, mirin, and honey (if using) until silky. Season with salt. It should taste like dessert with a savory edge.
• Spread into a buttered 9" × 13" dish. You can make it a day ahead up to this point and refrigerate it; add 10 minutes to the bake.

Top and bake

• Toss the panko, black sesame, brown sugar, melted butter, and salt together.
• Heat the oven to 375°F and bake the base 20 minutes until hot through.
• Toss the mochi cubes with the melted butter and scatter them over the top, leaving a little space between them because they puff and spread. Sprinkle the sesame crunch in the gaps.
• Bake 12–15 minutes until the mochi puffs, then broil 1–2 minutes, watching closely, until it's blistered and golden like toasted marshmallows.
• Drizzle with kuromitsu, dust with kinako if using, and finish with flaky salt. Serve hot, while the mochi is stretchy.