Preheat the oven to 180°C. Line a baking tray with non-stick paper or a silicon mat. Place the egg whites, sugar, coconut, salt and vanilla extract in a bowl and mix to combine. Using a tablespoon to measure, roll into walnut-sized balls. Place onto the prepared baking tray and flatten to a maximum of 1cm.
Bake for 10 minutes or until golden brown and toasted on the outside. Remove from the oven and allow to cool.
To make the chocolate ganache filling, place the cream in a saucepan over a medium heat and bring almost to the boil. Remove from the heat, add the chocolate and butter and stir until smooth. Refrigerate until cool and spreadable.
To assemble, spread a macaroon with a teaspoon of chocolate filling and then sandwich together with another macaroon.
If you find that your macaroon mixture is slightly dry and crumbly, making it difficult to work with, mix in 1 or 2 teaspoons of water until you have the right consistency.
For neater sides, you can push a tablespoon of dough into a round cookie cutter and then gently press it out onto the baking tray.
If you can manage not to eat them all in one sitting, you can store them in a sealed, airtight container for up to 5 days.