Place a medium ovenproof cast iron pan in the centre of your oven and preheat the oven to 230°C, gas mark 8. Let the pan heat up for at least 25 minutes (prepare your toppings while you wait).
Blitz the eggs in a blender on a high speed until very frothy (approximately 1 minute). With the motor running, gradually stream in the milk, then 2 tbsp butter and blend for another 30 seconds.
Add the flour, corn flour, salt and pepper and blend just to combine (this batter doubles well if you want to scale up).
Carefully remove the cast iron pan from the oven and swirl in the remaining 1 tbsp butter to coat.
Immediately pour the batter into the pan (the butter will brown quickly, so have batter at the ready to prevent burning).
Bake the pancake until it has puffed and browned around the edges – for approximately 20-25 minutes. It will begin to deflate as soon as it comes out.
Quickly top with the smoked salmon, sour cream and avocado and serve immediately.