Pierce 4 sweet potatoes with a fork. Microwave until softened, 12 minutes. Melt 3 tablespoons butter in a saucepan over medium heat and cook until brown flecks appear. Add 3/4 cup milk, 1/2 teaspoon kosher salt and 1/4 teaspoon vanilla; heat until warm but not boiling. Peel the potatoes and puree in a food processor with the warm milk, adding up to 1/4 cup more milk if needed. Top with lemon zest.