Adjust oven rack to middle position and preheat oven to 400 degrees F. Line a baking sheet with parchment paper. In the bowl of a food processor combine rye flour, flour, baking powder, and salt and pulse five times to combine. Add butter and pulse until it is the size of peas, about another 5 pulses. Add buttermilk and pulse until dough comes together into a ball. Shape dough into an 8-inch round. Bake until brown, about 40 minutes.
