Heat oven to 375 degrees F.
In bowl of stand mixer (or mixing bowl), whisk together all dry ingredients, with the exception of the walnuts and Turbinado sugar.
In separate small mixing bowl, lightly whisk together pumpkin puree, buttermilk and vanilla extract.
Add chopped butter to the flour mixture and beat with paddle attachment until well-blended, but you can still see pea-sized bits of butter.
Add buttermilk mixture to flour mixture and blend on slow speed until just moistened.
Turn mixture out onto floured surface. Shape dough into round disk (about a 6-inch diameter). Press walnuts into top and sprinkle with Turbinado sugar.
Transfer to a greased baking sheet.
Cut like a pie into 8 pieces, and create about a 1/4 inch space between slices.
Bake for about 20 minutes, until just starting to turn golden brown.