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Whisk together the egg whites and caster sugar in a large bowl for 2-3 mins until light and frothy, and the sugar has dissolved. Add the coconut, a pinch of salt and the vanilla, then stir until combined. Leave to stand for 10 mins.
Heat the oven to 170C/150C fan/gas 3½. Line a baking sheet with baking parchment. Scoop teaspoonfuls of the mixture and roll into compact balls, then arrange on the prepared baking sheet. Bake for 10-12 mins until golden.
Leave on the baking sheet to cool completely. Melt the chocolate in a heatproof bowl over a pan of simmering water, making sure the bowl doesn't touch the water, or in short 20-second bursts in the microwave. Tip the melted chocolate into a small bowl, then dip the bottom of each cooled macaroon into the chocolate and wipe off any excess. Arrange on a sheet of baking parchment, chocolate-side up, then put in the fridge for 20 mins, or until set. You can use any remaining melted chocolate to pipe in zigzags over the top, if you like, then leave to set.