Preheat your oven to 180°C -165°C (Fan)/Gas mark 4.
Prepare 8 inch/20 cm square tin (at least 2 inch/5 cm in height) by greasing inside and lining the bottom with baking parchment.
Place a large bowl over a pan of simmering water ensuring the water does not touch the bottom of the bowl.
Melt the butter, Tia Maria, orange juice, rind and chocolate in the bowl, stirring gently until fully melted.
Once melted, remove the bowl from the pan and allow to cool for 2-3 minutes.
Using a spoon stir in the sugar, vanilla and cocoa powder until well mixed.
Add the eggs one at a time, stirring between each addition.
Add the flour, salt, chocolate chips, hazelnuts and mix until you have just combined all the flour into the batter.
Pour the runny batter into the prepared baking tin and use a knife or spatula to spread it evenly around the tin and into the corners.
Place the tin into the centre shelf of the pre-heated oven for at least 30 minutes.
After 30 minutes, the brownies should be gooey in the centre. Check to see that there is a cracked crust on the top. Use a knife and cut a line through the centre, push aside slightly and check to see if the batter is still runny. If the batter is runny return the brownies to the oven and check again after a further 10 minutes. Once the brownies hold their shape you can remove from the oven, but they should still be sticky.
Once you have removed the cooked brownies from the oven, cut them to size in the tin. Allow to cool in the tin. Cut out and dust with icing sugar to serve.