Kentang Tumbuk Krim dengan Shio Koji

Bahan-bahan (4 porsi)

• 1½ lb russet and Yukon Gold potatoes ((2 large potatoes should be about 1½ lb))
• 1 tsp Diamond Crystal kosher salt ((for boiling the potatoes; use it if you do not use Shio Koji))
• 2 Tbsp unsalted butter
• ¼ cup heavy (whipping) cream
• 2 Tbsp shio koji ((you can replace it with ½ tsp kosher/sea salt))
• freshly ground black pepper
• 1 Tbsp unsalted butter
• a few stalks chives
• freshly ground black pepper