Preheat the oven to 350°F. In the bowl of a food processor add the oats. Blend until the oats are partially broken down – it should be a mix of small flakes and fully ground oats. Add the flour, brown sugar, baking soda, and kosher salt. Pulse a few times to mix the dry ingredients. Cut the cold butter into small pieces and add to the food processor. Pulse until the mixture resembles small pea-sized pieces.
Transfer the dry ingredients to a mixing bowl. Add the water and stir together. If necessary, use your hands to form the dough.
Line two baking sheets with parchment paper and set aside. Transfer the dough to a lightly floured surface and dust with more flour. Roll out to ¼-inch thickness. Cut the oatcakes out into circles and transfer to the prepared baking sheets. Gather the scraps of dough together and roll out again, repeating until all the dough has been used.
Bake the oatcakes for 12 to 15 minutes until they are lightly browned around the edges and the tops spring back when touched. Slide the parchment with oatcakes onto the counter or a rack to cool.
Serve these oatcakes on their own or with toppings such as cheese, jam, or savoury spreads. Enjoy!