Best started the day before you want to bake them, but not essential.

Method:

• Beat the butter in a stand mixer fitted with the paddle attachment until smooth and creamy, about 2 minutes. Add both sugars and continue to beat until fluffy. Add the eggs and vanilla and continue to beat for a few more minutes, periodically scraping down the sides of the bowl.

• Mix the flour, baking powder, bicarb and salt in a medium bowl and spoon into the mixer. Beat on the lowest speed and scrape down the sides of the bowl until the batter is smooth, about 1 minute.

• Add the chocolate and crisps and beat on the lowest speed until just combined. Do not overbeat or you’ll get potato dust instead of crisps. Pat the dough into a 5 cm rectangle, wrap in clingfilm and refrigerate overnight. Trust me on this. It really improves the texture of the cookies.