Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.

In a food processor, combine rye flour, all-purpose flour, baking powder, and salt.

Pulse the ingredients five times to combine.

Add cold diced butter to the mixture.

Pulse until the butter is the size of peas, about 5 pulses.

Add buttermilk to the mixture.

Pulse until the dough comes together into a ball.

Shape the dough into an 8-inch round on the prepared baking sheet.

Bake in the preheated oven for about 40 minutes, or until golden brown.