Grind steel-cut or rolled oats in a food processor or blender until it is the texture of cornmeal. (Skip this if you're using the already ground Scottish oatmeal.) Measure out 1⅔ cups (235g), and set aside the extra for dusting.
Pour the ground oats into a mixing bowl and add the butter, salt, and baking soda. Gradually stir in enough boiling water until a pasty, but not sticky, dough is formed and the mix is well blended.
Turn out the dough onto a surface lightly dusted with some of the extra oatmeal. Knead gently a few times, then roll out the dough to 3-5 mm thick. Cut the dough into rounds with a 2½ inch cutter, re-rolling the scraps as needed.
Heat a 10-inch cast iron skillet over medium heat until hot, then lightly butter it and evenly space 6-7 oatcakes in it. Cook them for 10-15 minutes on each side, then remove to a wire rack to cool. Repeat with the remaining ones.
Alternatively, place them on a cookie sheet and bake in a 375°F oven for 30 minutes, flipping them after 20 minutes.
Serve them warm or room temperature with butter, jam, cheese, chutney, or any desired topping.