Fill a large pot with about 2 inches of water and add a pinch of salt. Set steamer basket in pot, making sure water doesn't seep through holes. Bring to a boil, then reduce to a rapid simmer.
Add potatoes to steamer basket and steam until tender when pierced with the tip of a paring knife, 25 to 30 minutes.
Heat butter and milk in a small saucepan over medium heat until the milk is warm and the butter has melted. Remove potatoes from steamer basket. While potatoes are still hot, use a ricer to rice potatoes into a large bowl. Add milk mixture and gently combine with a spatula; season with salt and pepper. Serve immediately.