Cream together the butter and the icing sugar, then gently mix in the other ingredients until it comes together. Wrap in clingfilm and chill for at least an hour.
Preheat your oven to 180 degrees Roll the pastry out to the size of your tart tin and line the tin with it.
Bake the pastry case lined with baking paper and filled with baking beans for ten minutes. Then remove the baking beans and baking paper and bake for a further ten minutes. Leave aside to cool.
Make the ganache:
Heat the cream gently in a pan until it begins to simmer (or use the microwave). Place the chocolate pieces in a large bowl then pour over the cream. Leave it to sit for five mins approx then stir well until smooth.
Pour the ganache into the cool pastry case and chill the tart in the fridge until set.
Make the mirror glaze:
Make the mirror glaze following the directions in the link above. Allow the glaze to cool then pour the glaze over the top of the ganache. Chill in the fridge until set.
Chop some squares of dark and white chocolate. Slice some Maltesers in half. Decorate the top as desired.
The chocolate pastry recipe is from Teatime in Paris by Jill Colonna.