Step 1

Prepare a grill for medium-high heat. Bring potatoes, salt, and 1 quart water to a boil in a medium saucepan over medium-high heat. Reduce heat and simmer until tender, 12–14 minutes; drain. Grill potatoes, turning often with tongs, until lightly charred, 6–8 minutes.

Step 2

Meanwhile, set a medium skillet, preferably cast iron, on grill. Place butter and miso in skillet and cook, stirring, until butter is melted and mixture is smooth, about 2 minutes. Remove from heat; stir in garlic and vinegar. Season with pepper.

Step 3

Transfer potatoes to skillet, breaking some in half with tongs. Toss to coat in miso butter. Add parsley; toss again.