1. Preheat oven to 180 C / Gas 4. Lightly grease baking trays or line with baking paper.
2. Cream 175g butter with the dark brown soft sugar until light and fluffy. Beat in the 2 eggs and mix well. Stir in the salt, cinnamon, baking powder, boiling water, bicarbonate of soda and flour. Mix to combine. Stir in the Quaker Oats.
3. Drop cookie dough by spoonfuls onto the prepared baking trays. Bake at 180 C / Gas 4 for 10 to 12 minutes. Let cool before making sandwiches with the filling.
4. To make the filling: Beat egg white. Mix in the milk, 1 teaspoon vanilla and half of the icing sugar. Beat in the butter or margarine and remaining icing sugar. Beat until light and fluffy.
5. Spread filling on flat side of one cookie, then top with the flat side of another.
These are best enjoyed the same day.