Add the brown sugar and melted butter to a mixing bowl and stir until the sugar is dissolved
Stir in the milk before adding the pistachios, flour and baking soda. Fold this in until you have a well combined cookie dough then cover and set in the fridge to chill while making the filling
Melt two thirds of the white chocolate and save the rest for later. Add the melted chocolate to a bowl with the pistachio butter, mix until combined then set aside to cool until hardened but still slightly pliable
Divide your cookie dough into 8 equal pieces. Take a piece, divide it in half and flatten out each half. Hold one half in your hand then scoop an eighth of the filling into the centre. Lay the other half on top, press to seal so the chocolate doesn't leak and roll into a ball
Repeat this with each piece then lay the balls out on a lined baking tray, making sure to leave enough room between each one as they will spread out a lot in the oven
Lightly press the remaining white chocolate into the top of each ball then place in the oven to bake at 200°C/180°C fan for 10-12 minutes until lightly golden then remove and let them cool for 10-15 minutes before serving
