Pre-heat the oven to 180°C / 160°C fan assisted / gas mark 4 / 350°F
Line a (6 inch) square cake tin with baking parchment up the sides and hanging over the sides, clip to secure with mini bulldog clips. Lightly grease the parchment.
Cream the butter and sugar together until light and fluffy.
Add the eggs one at a time and mix until combined.
Whisk together the coconut, chopped pistachios, glacé cherries and cocoa powder, making sure the cherries in particular are broken up and even distributed.
Mix the coconut pistachio mixture into the butter and sugar mixture until completely combined.
Pour the batter into the prepared baking tin, smoothing the top. Then bake for 40 minutes.
Leave the chocolate pistachio slice in the tin to cool for 15 minutes before removing and covering with chocolate.
To make the chocolate topping, break up the chocolate and place in a heatproof bowl along with the butter and salt. Melt in the microwave for 1 minute, removing and giving a stir, then microwave for a further 20 seconds. Stir until the chocolate is melted and glossy.
Pour the chocolate over the slice. Scatter the extra pistachios and desiccated coconut, over the top.
Then allow to cool completely the chocolate sets before cutting into bars.