Browning the butter with spices lends a deep, nutty flavor to this Southern classic.
Preheat oven to 350°F. Fit each piecrust into a 9-inch pie pan. Prick the bottoms of the piecrusts several times with a fork. Line piecrusts with parchment paper, and fill with pie weights or dried beans. Bake in preheated oven until slightly golden, about 15 minutes. Remove weights and parchment, and cool completely, about 1 hour.
Preheat oven to 350°F. Melt butter in a small saucepan over medium. Add cinnamon stick and cloves; cook, stirring occasionally, until foam subsides and butter begins to darken and smell fragrant, 3 to 4 minutes. Remove from heat, and pour through a fine wire-mesh strainer over a bowl. Discard cinnamon stick and cloves.
Beat brown butter, sweet potatoes, condensed milk, sugar, eggs, vanilla, lemon juice, and salt with an electric mixer at medium speed until light and fluffy, 2 to 3 minutes. Divide mixture evenly between 2 piecrusts. Bake on middle rack in preheated oven until filling is slightly puffed and set, about 45 minutes. (If the crust gets too dark, loosely cover with aluminum foil.) Cool at least 1 hour before serving.
Note: If your sweet potatoes are at all stringy--which can happen sometimes, especially with larger sweet potatoes--you will need to pass them through a food mill or sieve.