Cooked, pureed peas give these mashed potatoes their unique color and a hint of sweetness.
Peel and cut potatoes crosswise into 1 1/2-inch-thick slices. Place slices in a medium saucepan, and cover with cold water. Bring to a boil over medium heat; add 1 tablespoon salt. Reduce to a low simmer, and cook until potatoes are tender when pierced with a knife, about 15 minutes. Transfer to a colander; let drain.
Meanwhile, melt 1 tablespoon butter in a small saute pan over medium-low heat. Add peas, and cook until they are tender and bright green, 4 to 5 minutes. Transfer to the jar of a blender, and add milk; blend until mixture is smooth and combined.
While still hot, pass potatoes through a ricer or food mill into a large heat-proof bowl. Stir with a wooden spoon until they are smooth, about 1 minute. Using a whisk, incorporate remaining 4 tablespoons butter. Whisking constantly, add pureed pea mixture, and season with salt and pepper. Serve immediately, or keep warm over a pan of simmering water.