Prick sweet potatoes several times with a sharp knife. Place on a 12- by 15-inch baking sheet and bake in a 375° regular or convection oven until very soft when pierced, about 40 minutes. When cool enough to handle, peel sweet potatoes and whirl in a blender or food processor until smooth. You will need 1 cup purée; save any extra for other uses.
In a large bowl, with a mixer on medium speed, beat butter and sugar until well blended. Add eggs one at a time, beating to blend after each addition. Beat in 1 cup sweet potato purée, the orange peel, and the vanilla.
In another bowl, mix flour with baking powder, mace, and salt. Add the flour mixture to batter; mix on low to incorporate dry ingredients, then beat at medium speed until well blended.
Scrape batter into a buttered 4 1/2- by 8 1/2-inch loaf pan.
Bake in a 350° regular or 325º convection oven until a wooden skewer inserted into the center comes out clean, 1 hour to 1 hour 15 minutes. Cool in pan on a rack about 15 minutes. Invert cake onto a plate and cool completely, about 1 hour. Cut into 1/2-inch slices and serve, or wrap tightly and store at room temperature.