Put the butter into a large mixing bowl and use an electric whisk to beat it. Add the icing sugar, and mix that through, followed by the egg, vanilla and salt. When the mixture is smooth, add the flour, give it a quick stir with a wooden spoon then use the electric whisk to mix it all through.
Turn the dough onto a floured worktop and bring it together with your hands. If the dough is too sticky, you can place it into the fridge at this point, otherwise, roll the rough out to about 1 cm thick and use a 4cm cookie cutter to cut out rounds of dough. Place the rounds onto a baking tray and place into the fridge for 30 minutes.
Preheat oven to 180 C / 160 C fan and place the cookies into the oven to bake for 15 - 20 minutes, until cooked through but not coloured.
Remove the cookies from the oven and allow to cool.
When cool, pipe a ring of vanilla buttercream on each cookie, smoothing the inside of the ring with a spatula or spoon. Press the toasted coconut into the buttercream to create a nest effect then finish by adding the mini eggs into the nest.