Preheat oven to 425F and line a baking sheet with parchment paper.
In a large bowl, beat together butter, sugar and salt. Beat in egg, then the mashed sweet potato. Stir until smooth.
Sift in flour and baking powder. Stir until flour is completely mixed inches Dough should be slightly sticky, but not wet. If it is wet (i.e. if your sweet potatoes were unusually moist) add an additional tbsp or two of flour.
Turn dough onto a lightly floured surface and knead three or four times before shaping into a rectangle and rolling out to 3/4 inch thickness. Use a 2 1/2 inch cookie cutter to cut rounds. Reroll dough once and continue to cut rounds. Place all on prepared baking sheet.