Step 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).

Step 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.

Step 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.