1. Preheat the oven to 200°C. Line a 9-inch square cake pan with parchment and coat lightly with cooking spray.
2. Whisk together the pumpkin, buttermilk and egg yolk in a small bowl until smooth; set aside.
3. Grind the oats in a food processor until they form a coarse texture. Add the flour, brown sugar, baking powder, pumpkin pie spice, baking soda and 1/2 teaspoon salt and pulse until combined.
4. Add the butter and pulse until the mixture looks fine. Add the cranberries and pulse once. Add the pumpkin mixture and pulse until the oat mixture is just moistened, (3 or 4 pulses).
5. Turn the dough out of the processor bowl and gently knead until combined but not overworked.
6. Gently press the dough into the prepared cake pan so it is about 7 inches across and 3/4 inch thick (it won't go all the way to the edges). Sprinkle the top with the brown sugar.
7. Bake until the top begins to crack and is golden and firm to the touch, 25 to 30 minutes.
8.Remove from the oven and let cool in the pan for 10 minutes, then tip out onto a cutting board and slice into 8 scones. Transfer the scones to a cooling rack and let cool for 15 minutes before serving.