Peel and cut both the russet and Yukon Gold potatoes into evenly sized chunks (about 2 inches) so they cook at the same rate. Soak the cut potatoes in cold water for 30 minutes to remove excess starch, then drain and rinse thoroughly under cold water. Place the potatoes in a large pot and cover with cold water, adding 1 tsp kosher salt. Bring to a boil over medium-high heat, then reduce to medium and simmer for 15-20 minutes until completely tender when pierced with a fork. I like using both potato varieties because the russets give fluffy texture while the Yukon Golds add creaminess—they complement each other beautifully.

While the potatoes are cooking, combine the 2 tbsp unsalted butter, 1/4 cup cold heavy cream, and 2 tbsp shio koji in a small saucepan over low heat. Stir gently until the butter is melted and the shio koji is fully incorporated and warm. The shio koji adds wonderful umami depth and a subtle salinity, so resist the urge to add extra salt—the mixture should taste balanced and warming. Keep this on very low heat so it stays warm but doesn't break or separate.

Drain the cooked potatoes thoroughly in a colander. Return them to the pot and place it on low heat for 1-2 minutes, stirring gently, to evaporate any remaining moisture. This step is crucial—excess water will make your mashed potatoes gluey instead of fluffy. Once they're dry, remove from heat and begin mashing with a potato ricer or masher until you reach a creamy, lump-free consistency.

Pour the warm butter and shio koji mixture from Step 2 over the mashed potatoes and fold gently but thoroughly until fully incorporated and creamy. Taste and adjust seasoning with black pepper to your preference—the shio koji already provides the salt component, but add more pepper as needed for depth. I find that a generous crack of fresh black pepper really makes these stand out, so don't be shy.

Transfer the mashed potatoes to a serving bowl and create a small well in the center. Top with 1 tbsp unsalted butter and a generous sprinkle of freshly chopped chives and black pepper. Serve immediately while hot, allowing the butter on top to melt slightly into the potatoes as guests serve themselves.