Start to finish: 25 hours (1 hour active)
¼ cup fresh buttermilk with live
2 cups heavy cream
2 teaspoons brown mustard seeds
2 teaspoons yellow mustard seeds
4 pounds Yukon gold potatoes, peeled and halved
In a clean 2-cup liquid measuring cup, combine the cream and buttermilk. Cover tightly and let sit at room temperature for 24 hours.
To make the butter and buttermilk, place the cream mixture in the bowl of a food processor. Process on high. The mixture will initially thicken into a whipped cream, then it will separate into a liquid and solid. This process can take several minutes of processing, so be patient.
Once the mixture has separated into a solid (the butter) and a white liquid (the buttermilk), set a strainer over a bowl and pour the mixture through it. Set aside the bowl of buttermilk.
Fill a medium bowl with ice water and place the butter in it. Use your hands to gently knead the mixture together to create a large lump of butter. Remove the butter from the water and add fresh ice water to the bowl. Repeat the kneading process until the water is clear. Remove the butter from the water and set aside in a clean, dry bowl.
With a mortar and pestle, crust the brown and yellow mustard seeds. Stir the crushed seeds into the butter. Taste and season the butter with salt. Set aside.
Heat the oven to 350 F.
Place the potatoes in a large pot, then add enough water to cover by 1 inch. Salt the water, then bring to a boil. Cook until tender when pierced with a fork, then drain and transfer to a sheet pan. Bake for 10 minutes.
Using a food mill or a ricer, process the potatoes into a large bowl. Stir in ¾ cup of the reserved buttermilk, 2 tablespoons of the mustard butter and salt, to taste. Serve with additional mustard butter on the side.
Nutrition information per serving: 390 calories; 210 calories from fat (54 percent of total calories); 23 g fat (14 g saturated; 0.5 g trans fats); 85 mg cholesterol; 42 g carbohydrate; 3 g fiber; 2 g sugar; 7 g protein; 270 mg sodium.