Remove the pan or mould from the oven and place the butter inside. Use a pastry brush to spread it or rotate it to cover the base and sides.
Pour the batter on the centre.
Spread out the apple slices on the batter and return to the oven for 15-20min. You should see the sides expand up the sides and the dutch baby should be golden.
You can unmould it or serve it directly in the pan or mould with plenty icing sugar and ice cream (the ice cream is optional).