Whip the butter, sugar, and icing sugar using a stand mixer, electric handheld mixer, or whisk until the butter is light. fluffy and aerated.
170 grams butter, 60 grams icing sugar, 100 grams caster sugar
Add in the flour, salt, and chocolate chips. Mix with a spatula or wooden spoon until a soft pliable dough forms. Knead the dough 3 to 5 times bringing the dough together in all.
240 grams plain flour, ½ teaspoon table salt, 75 grams dark chocolate chips, 75 grams milk chocolate chips
Line a 9"x9" square baking tin for a 23cm tart tin with parchment paper. Press the dough into the baking tin. Dock or prick the dough with a fork.
Place the biscuits in the fridge for at least 30 minutes before baking.
Preheat the oven to 180°C/355F and bake the biscuits for 20-30 minutes, until the biscuits are a nice golden brown. Remove from oven and cool on a wire rack before serving.