1. Preheat oven to 375ºF; line a large, rimmed baking sheet with foil. Prick potatoes all over with a sharp knife and place on baking sheet. Bake until potatoes are tender, 45 minutes to 1 hour. Set potatoes aside until cool enough to handle, then scoop out flesh and mash in a bowl. Measure 3/4 cup of puree for biscuits and place in a bowl (cover and refrigerate any remaining puree for another use).
2. Raise oven temperature to 400ºF. Add buttermilk to puree and stir until smooth. Sift flour, ginger, cinnamon, baking powder, baking soda and salt into a large bowl. Using a pastry blender or your fingers, mix in 6 Tbsp. butter until mixture resembles coarse crumbs. Add potato mixture and gently mix until dough just begins to come together, adding more buttermilk 1 Tbsp. at a time if necessary. Turn dough out onto a lightly floured surface and knead gently. Pat out dough to a 1/2-inch thickness and use a 1 1/2-inch round cutter to cut out as many biscuits as you can. Place biscuits 1 inch apart on a large ungreased baking sheet. Gather scraps, pat together, cut out more biscuits and place on baking sheet. Repeat until all dough has been used.
3. Melt remaining 1 Tbsp. butter and brush on biscuit tops. Bake for 12 to 14 minutes, until golden. Let cool on wire racks for 5 minutes to serve warm or longer to serve at room temperature.