13g butter
13g flour
83ml hot milk
33g grated cheese (ideally gruyere-style)
1 egg yolk
1/6 tsp mustard powder
2 egg whites
Method
Brush 2 large oval ramekins with butter and dust with flour and keep in fridge.
Melt butter, add flour, cook roux for 1min over low heat. Beat in hot milk and cook for 2-3min more on low heat. Stir in cheese, egg yolk and mustard. Keep warm while whipping egg whites with a little salt (to soft peaks).
Fold in cheese sauce one-third at a time. Don't overmix.
Cook in air fryer at 180 for no more than 10 minutes. (Remove plate to be sure soufflé lids don't get caught on the top as you take them out.)