Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt.
Add the cold, cubed butter to the dry ingredients. Using your fingers or a pastry cutter, work the butter into the flour until the mixture resembles coarse crumbs.
Slowly pour in the cold milk (or water), and stir with a wooden spoon until the dough just comes together. Be careful not to overmix.
Turn the dough out onto a lightly floured surface and gently knead it for about 1 minute, just enough to bring the dough together and smooth out any rough spots.
Flatten the dough into a round shape, about 1-inch thick, and place it on the prepared baking sheet.
Bake the bannock in the preheated oven for 20-25 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Remove the bannock from the oven and let it cool for a few minutes before serving.