Preheat oven to 450°F/230°C. Line a baking sheet with parchment paper or a silicone baking mat.
In a large bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, nutmeg, and sugar. Cut the cold butter into the flour mixture until the butter pieces are about the size of a pea or slightly smaller.
Stir in the buttermilk and pumpkin until just combined. Do not over mix. Dough should come together and will be thick.
Scrape the dough out of the bowl and onto a lightly floured surface. Gently pat the dough with your hands until it is about 1/2 to 1 inch thick. Then fold the dough in half or thirds and pat down again, repeating 4 or 5 times. Final thickness should be about 1 inch.
Use a round biscuit cutter to cut the dough into circles. The edge of the cutter should be thin and sharp - press straight down firmly into the dough, then lift the cutter straight up and repeat. Pat the remaining dough together again and cut more biscuits, until dough is all cut.
Place the biscuits on the prepared baking sheet, with the biscuits nearly touching each other. This helps the biscuits to rise up instead of out. If possible, chill the biscuits for 5 to 10 minutes.
Bake for 10-12 minutes until the biscuits are lightly golden on top and bottom, but take care not to over-bake. Serve immediately.
Avoiding over-mixing the dough. The patting and folding process, and keeping the dough cold will all help the biscuits to be flaky and tender.