Place a 9"-12" diameter pan (frying pan or baking pan) in the oven before preheating to 425°F. The pan should be as hot as the oven when you pour the batter into it.
While the oven is heating, make your Yorkshire pudding batter. Whisk together the oat flour and pinch of salt in a medium sized bowl.
Crack the eggs into the bowl and mix well with the dry ingredients to a smooth paste, then blend in the milk.
When the oven has reached 425°F, take the pan out of the oven and drop the butter or oil into it. Wearing oven mitts, pick up pan and swirl butter around to coat the bottom of the pan. Once melted, immediately pour the batter on top.
Place back in the oven and bake for 20-25 minutes without opening the oven door, until puffed and golden brown.