An oven thermometer is essential for consistent results.
Dough is always better chilled and rested.
Measure ingredients by weight not volume using electric scales.
Use an electric or hand whisk to cream together 'butter' and sugar, you'll know when it's done if you roll some of the mixture between your fingers the sugar should be completely dissolved.
Add ground coffee to chocolate items to enhance the flavour (they won't taste like coffee).
If you don't have a baking tin the recipe suggests use a tin conversion chart.
Do not substitute a baking block or butter with margarine in dough, it contains too much water.