Preheat the oven to 180C and grease a 12 hole muffin pan.
Sieve the icing sugar and the ground almonds into the bowl of a food mixer.
Melt the butter over a low heat and cool to lukewarm.
Add the pistachio paste to the icing sugar and ground almonds. Add the eggs one at a time, mixing with the paddle attachment until fully incorporated. You may need to scrape the bowl down between adding the eggs so that this is all mixed in.
Pour in the melted butter and mix until incorporated, this will take a few seconds.
Spoon the batter into a disposable piping bag and fill the mould 3/4s full, or use a spoon.
Gently place the square of chocolate in the middle of each cake.
Coarsely chop the pistachios in a food processor or with a large knife.
Scatter them over the small cakes.
Bake for 20 minutes or until golden and a cocktail stick comes out clean.
You can eat the cakes slightly warm, or wait until they are cool and because of the ground almonds they will stay really moist.